He will understand that I was not joking.

He will understand I was not joking.

Which of the sentences is correct? Are there any specific rules about the use of "that" in the sentences I reported as example?

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    They are both correct, but I don't know the technical terms here so I'll leave it for someone else to answer.
    – mmyers
    Commented Aug 16, 2010 at 22:28
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    Stylistically speaking, my preference is to remove "that" when it is not necessary; it's verbal fluff and not good for much aside from padding word count.
    – cori
    Commented Aug 17, 2010 at 6:02
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    @cori +1 very prudent advice. Also, when speaking, "that" can make a good filler word when one is trying to think of what to say next. A bit like saying "aaah" or "ummm". Commented Aug 17, 2010 at 7:09
  • 3
    > This is the farmer that grew the corn that fed the cock that crowed the morn that woke the priest all shaven and shorn that married the beggar all tattered and torn that kissed the maiden all forlorn that milked the cow with a broken horn that tossed the dog that chased the cat that killed the rat that ate the malt that lay in the house that Jack built. Commented Aug 17, 2010 at 7:19

7 Answers 7


That can almost always be dropped. In your example, that is being used as a conjunction, i.e. it is introducing a subordinate clause as the object of the main sentence. In most situations where this is the case, it can be dropped. I cannot think of any where it can't be dropped.

When that is used as a demonstrative pronoun, e.g. "that was a nice question," it must be kept or replaced with another pronoun, e.g. "yours was a nice question."

When used as a relative pronoun, it can usually be dropped. For instance, "several people read the question that you wrote" can also be "several people read the question you wrote". But if used in a question with who, it should be kept. For example, "Who was the person that wrote this question?" cannot be *"Who was the person wrote this question?".

I'm sure I missed something, but the comments should keep me honest.

  • 14
    I would add that "that" usually cannot be dropped when it's an adverb (meaning "very", "so much", "really", "particularly"). If you drop "that" in the sentences "I'm not that stupid" or "it wasn't that difficult", you get a slightly different meaning (I'm not stupid at all, it wasn't difficult in any way). And if you drop "that" in the sentence "I was that happy I started to cry", you get a completely different meaning (I enjoyed crying). All three examples are still perfectly grammatical without "that", but the meaning is different.
    – RegDwigнt
    Commented Sep 1, 2010 at 14:44
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    As for the third paragraph, the relative pronoun “that” cannot be omitted if it is used as the subject in the relative clause. It has nothing to do with the word “who” in the main clause. For example, “that” cannot be omitted in the sentence “Where are the eggs that were in the basket?” or “The eggs that were in the basket are now in the fridge.” (The first example is taken from usingenglish.com/forum/ask-teacher/…) Commented Oct 28, 2010 at 18:52
  • 1
    What about Who's the girl that kissed you this morning? The that can't be deleted there because it's the subject. And that can be the subject of just about any restrictive relative clause. The important point is that the subject slot hasta be filled in a tensed clause; infinitives and gerunds frequently have only implied subjects (Bill told Mary to take out the garbage vs Bill promised Mary to take out the garbage). This has nothing to do with Who-questions, incidentally. Commented Nov 22, 2014 at 14:49
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    @JohnLawler That explanation won't quite work, or at least with relative clauses. Reason is that we can delete the subject of any tensed embedded clauses within the relative clause. For example: "That's the man they say stole the Mona Lisa" Commented Aug 13, 2015 at 13:34
  • 1
    Sort of. There's a set of rules called "cyclic" rules, which apply bottom up from the lowest clause, but may be reapplied in a higher clause if their conditions are met. Cyclic rules are governed by predicates and structures in one clause, in most cases; Passive is an example. Then there are noncyclic rules, which can move material from arbitrarily far away in the sentence, as long as it isn't across an island boundary, like Relative Clause Formatoin. Material affected by a rule on a lower cycle becomes available to rules on a higher cycle. Commented Aug 14, 2015 at 13:44

For relative clauses: If the missing element (which is said to be "relativized") of the relative clause is the subject, "that" is obligatory, otherwise, it is optional:

Peter tasted the wine (that) his mother bought for him

Peter tasted the wine that/*() had cost only 12$.

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    The question is about a "noun clause" (AKA content clause). Comments about relative clauses are irrelevant. Commented Dec 25, 2011 at 17:25
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    @BrettReynolds But some of us didn't realize is was specifically a "noun clause" and found Arne's answer to be helpful with the more generic question, "When 'that' joins two clauses (regardless of type), under what conditions can (or should) be omitted?"
    – HTG
    Commented May 21, 2013 at 14:28

"That" is a conjunction in this type of sentence. (In "the cat that jumped over the wall", it is a relative pronoun.) It is in general OK to leave out the conjunction "that" now and then, as long as no ambiguity arises. Everybody does it all the time, even in formal style, though it happens more often in informal writing and speaking. It is possible too in German, though not in Dutch, as you probably know.

*She believed the judge, who was older than her father, was putting the moves on her.

In this sentence, the reader is led onto a false scent because "that" is omitted after "believed". This is a serious mistake. The reason why it is bad here is that "to believe" can have an object, so that the reader thinks "the judge" is simply the object of the verb, instead of the subject of a dependent clause, which in fact it is. When he finds out he is getting the sentence wrong, he needs to read back to repair the damage.

They say she was unable to realize her husband had left her.

This sentence is doubtful, because "that" is left out two times in a row. It makes the sentence a bit too loose, a bit messy: though the reader will probably get it right in one go, he will still need to spend a little more energy on it than necessary.


In the example you gave, "I was not joking" is a subordinate clause. One way to think about this is that there are two sentences

  1. He will understand X.
  2. X = I was not joking.

"that" is not exactly a conjunction as Vincent said. A conjunction joins two independent clauses together, but here 2. is sort of the object of 1. Any time you want to use a clause as the object of a sentence, you can optionally precede it with "that." However, if you want to use a clause as the subject of a sentence, it will always be preceded by "that"

That I was not joking will be clear to him.

*I was not joking will be clear to him.

Sometimes clauses modify nouns. In these cases, they're called relative clauses, and what pronoun appears before the clause varies. One of the most important factors of whether or not "that" or "which/who" precedes the clause is whether the clause is "restrictive". To modify somee examples from Wikipedia:

(Restrictive) The house that Jack built fell down.

(Restrictive) The house __ Jack built fell down.

(Non-restrictive) The house, which we all thought was in fine condition, fell down.

In the first two examples, the relative clause picked out a particular house from all possible houses. In these sentences, there is a tendency to use "that" or nothing. In the third example, the particular house was already determined, and the relative clause simply introduced extra information. These relative clauses tend to be preceded by a wh- word.

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    When introducing a subordinate clause, "that", is indeed a conjunction. mw2.merriam-webster.com/dictionary/that%5B2%5D Commented Aug 17, 2010 at 7:32
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    If you have to pigeonhole it into one of the traditional parts of speech, you have to call it a conjunction, but it doesn't behave like any other conjunction, so modern linguists don't call it one. Most often it's a "complementizer".
    – Colin Fine
    Commented Aug 17, 2010 at 10:11
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    The difference is that a conjunction simply joins two clauses together: "I went out with Mary and it was a lot of fun." But "that" subordinates the clause which follows it. That is, it turns an entire clause into the subject or object of a sentence. In my conjunction example, neither clause is the subject or object or anything of the other, making "and" simply a conjunction. I was, like Colin said, speaking from the perspective of modern linguistics.
    – JoFrhwld
    Commented Aug 17, 2010 at 14:43
  • "(Non-restrictive) The house, which we all thought was in fine condition, fell down." I concede to a strong dislike for the use of "which" rather than "that, except for the few essential times it is required. In this sentence, I still think "that" works quite okay: "The house, that we all thought was in fine condition, fell down."
    – Cargill
    Commented Nov 26, 2015 at 4:54

As most of the other answers here point out, including that in a sentence like "He will understand that I was not joking" is optional. But I'd like to offer some examples where that, despite performing the same basic function that it does in "He will understand that I was not joking," either is crucial for sense or vastly improves the flow of the sentence. My examples fall into four categories.


I appreciate [that] CAN-SPAM creates a single set of rules for an industry previously struggling with a hodgepodge of 34 existing state antispam laws.

Long Beach Opera recognizes [that,] in order to preserve and strengthen opera's significant in American culture, it must continue to develop new approaches in programming.

He claimed [that] customers complain [that] the new product is heavier than the iPad2 and lacks any major improvements from the old version.

In each of these sentences, readers can supply the implied that themselves, but with more effort than the author should require them to make.


But he insists [that] the charges are false, and that he's sending as many e-mail messages now as he did before the suit was filed.

Google says [that] new movies will be $3.99 and older ones $2.99, and that most rentals will be for 24 hours.

Political consulting firm Election Data Services, which tracks voting-machine usage nationwide, estimates that 40 percent of voters will cast ballots on touch-screen machines this year, and [that] about 42 percent will cast them either on optical-scan machines or on traditional paper ballots.

In these sentences, using that in one branch of a parallel construction calls for using it in another, for balance and clarity.

Ambiguity: Modifier Placement

You’d think [that] by now [OR HERE?] I’d be used to Windows’ mystifying default behaviors.

The ISPs promise [that,] under the new agreement, [OR HERE?] content providers will not be given your subscriber information such as your name, address, telephone number, and e-mail address.

A spokesman for the ISP says [that] online [OR HERE?] criminals are stealing AOL users’ account passwords and using the profiles to advertise their sleazy wares.

In these sentences, that could fall in either of two plausible places, and the meaning of the sentence would change somewhat depending on its placement.

Ambiguity: Attribution

Microsoft says [that] the transition to a 64-bit OS on the desktop will take a few years, and [HERE, TOO?] typical users will be on the far end of that distribution curve.

AOL says [that] it will offer online backups, but [HERE, TOO?] the feature won’t be included in the July 13 beta.

Providers of the service say [that] the bugs have been all but exterminated and [HERE, TOO?] the service is ready for prime time.

In these sentences, the ambiguity relates to whether both clauses are assertions made by the named source, or whether the first is from that source while the second is a follow-up remark made by the author.

  • 1
    Wow! Excellent examples, chapeau. So many people don't get this and just say "that is never required, period". Commented Mar 16, 2013 at 7:36
  • +1 for this answer and @joshua's. And I find that although dropping "that" can sometimes reflect better how native speakers talk, especially informally, it can also make reading more difficult for at least some non-native speakers. In particular, people used to a romance language can sometimes understand the sense more quickly when fewer "unnecessary" "that"s are dropped.
    – Drew
    Commented Feb 12, 2014 at 6:05

There are several different kinds of that in English (besides the ordinary demonstrative this, that, these, those). All of them are used in linking clauses, but they link different kinds of clauses, and follow different syntactic rules.

These that's are not, however, conjunctions; they are Complementizers. Complementizers are one of the parts of speech that have been discovered since the list of POS was drawn up in the 5th century. Not surprising, really; why should we use Roman science in the 21st century?

There are three different kinds of subordinate clauses, depending on what the clause is acting like:

  • Adverb clauses normally contain a subordinate conjunction.
    When I arrived, he was already here. He was already here when I arrived.
    That used to be a possible complementizer for all tensed clauses,
    as in Chaucer's Whan that Aprille ... hath perced ... 'When April has pierced'
    or Andy Griffith's before that we set up the tent in What it Was, Was Football

    That is used as a complementizer in tensed adverb clauses in Modern English only in local dialects like Griffith's North Carolina; that's what makes Chaucer sound strange. It is, however, used in tensed complements and relative clauses.

  • Noun clauses (also called Complements or Verb Complements) act like nouns, as subject or object for a predicate. The clauses are boldfaced below. There are four varieties, each with its own complementizer:

    Subject: For me to leave early would be a mistake. (infinitive complementizer)
    Subject: Leaving early is not recommended. (gerund complementizer)
    Subject: What he told me is not for publication. (embedded question complementizer)
    Subject: That I have to leave early is unfortunate. (that complementizer)

    Object: They told me to leave early. (infinitive complementizer)
    Object: I hate his playing the piano at all hours. (gerund complementizer)
    Object: I didn't hear what she told you. (embedded question complementizer)
    Object: They told me that I had to leave early. (that complementizer)

    Note that one of the complementizers is that, for tensed clauses.
    This that may be deleted, provided it is not the first word in a sentence.

  • Adjective clauses modify nouns or noun phrases. There are two kinds: relative clauses and NP complements; that occurs in restrictive relative clauses, and also in NP complements.
    Relative: the man that came to dinner (that is subject; cannot be deleted)
    Relative: the man that I saw in the station (that is not subject; may be deleted)

    NP Complement: the rumor that I have to leave early (NP complements are just complements that modify nouns formed from verbs or predicate adjectives.)
    It's possible that she needs a job ~ the possibility that she needs a job
    That in NP complements can be deleted, but not nearly as often as in verb complements.

  • 1
    I appreciate your erudition, and upvoted your answer, but I wonder if you could clear something up. Shakespeare is generally considered Modern English, and he used that as a complementizer all the time. So perhaps you mean small-m "modern" English, as in contemporary English? Or do you in fact mean big-M Modern English?
    – Robusto
    Commented Dec 20, 2013 at 22:39
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    Syntax has changed from Early Modern English (roughly 1550-1750) to Modern English. That's mostly what's changed, in fact, besides new senses for words. Commented Dec 20, 2013 at 23:37

If your sentence is unclear or difficult to parse for the listener/reader without that, then insert it; otherwise, feel free to omit it.

The understanding of your audience is always priority #1.

  • 4
    I would just note that in the former case, although you should feel free to omit it, you should also feel free to include it. The oft-cited advice to “Omit Needless Words” is just a suggestion, not a hard-and-fast rule that “needless words” must be omitted.
    – nohat
    Commented Nov 4, 2010 at 18:41
  • @nohat I agree. Commented Nov 4, 2010 at 19:34

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