A simple Google search reveals that the word that can be used in many ways: as a pronoun, determiner, adverb and conjunction. I'm wondering about its usage specifically as a conjunction.

Take the following two examples:

I think that I know who sings this song


I think I know who sings this song

From a colloquial standpoint, I would use the shorter version (the latter).

However, what is the grammatically-correct form? Is the word that, when used as a conjunction, necessary from a grammatical standpoint? It seems superfluous to me.

2 Answers 2


This "that" is not a conjunction;
it is a Complementizer.
It introduces tensed clauses.

Tensed clauses of several kinds, in fact:

In both cases, that may be deleted if it's not needed for clarity.
(there are several contexts where that may not be deleted, however; it's not totally optional)

  • Ah ha! I didn't know how to search for this kind of information - (relatively) obscure grammar concepts are hard to find through Googling... further reading can be done here; that page would have answered my question before it needed to be asked! Your other answer was also very helpful (unfortunately it didn't turn up when I searched English.SE). Wikipedia also says that complementizers are special subordinating conjunctions. Any thoughts? Jun 10, 2014 at 20:15
  • Total nonsense from people who hafta have some way to jam them into the Classical Eight Parts Of Speech somehow. Jun 10, 2014 at 20:25
  • Haha, I see. Well, that's good to know! There is a reason why there's 'Wikipedia general' and 'Stack Exchange specific' (in my eyes). Sometimes Wikipedia just doesn't cut it! Thanks :) Jun 10, 2014 at 20:27
  • I'm not sure SE cuts it, either. As you point out, it's hard to find that information here, too, if you're not looking for it and nobody points you in that direction. Plenty of people will be happy to give you their opinions on it, though. Jun 10, 2014 at 20:30
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    @automaton: "Please avoid extended discussions in comments. Would you like to automatically move this discussion to chat?" No, thank you, I wouldn't. I don't like the chat facilities here. Jun 11, 2014 at 2:28

There are definitely times when 'that' as a conjunction is necessary. One example would be with certain verb forms "I have heard he is dead." vs "I have heard that he is dead."

Another case might be when the subordinate clause marked by the 'that' comes before the main verb "That he was there, is certain, why he was there, unknown."

That is used as a conjunction in some cases when you are relating one clause to another: "He another a whole bowl of Ice Cream, after that he was full."

The OED gives another interesting example when using 'that' as a conjunction in a periphrastic: "It was because he failed to prove this that his case broke down."

Ultimately, that using the word 'that' as a conjunction is often superfluous cannot be denied, that much is sure. However, there are still many cases when it is definitely necessary.

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    'I have heard he is dead' may be used. 'I have heard you snore' may well have a different intended meaning from 'I have heard that you snore'. Jun 10, 2014 at 19:26
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    In your example ...after that he was full the word that is a pronoun (not a subordinator/complementizer/ conjunction). Jun 13, 2014 at 19:09
  • Strange, for some reason the OED listed that usage under the conjunction form of "that" but your explanation makes a lot more sense.
    – pavja2
    Jun 14, 2014 at 12:45
  • @EdwinAshworth: Because the first one is read not as the second one with an omitted "that", but as a different construction that is more like the Latin AcI. You see that when you put it in the third person: "I have heard that he snores" (that-clause), "I have heard he snores" (the same with omitted "that"), "I have heard him snore" (AcI). Nov 28, 2017 at 18:24

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