15

Sometimes, the word 'that' to introduce a dependent clause is optional. For example, these sentences both make sense with or without 'that':

Long books [that] religious people like tend to be Bibles.

Water tanks [that] fish need are spacious.

... whereas in these sentences, 'that' is mandatory and the sentence is ungrammatical without it:

Those that are rotten must be thrown away.

Cars that break down endanger pedestrians.

I can't quite put my finger on the rule which determines when 'that' must be used. What is it?

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  • 3
    I noticed right away that in your last two examples, "that" is followed by a verb, which is not the case with the first two.
    – Kit Z. Fox
    Jul 25 '11 at 17:55
  • 1
    Related: Use of "that" in a sentence.
    – apaderno
    Jul 25 '11 at 18:26
  • Embedded clause or dependent clause? Jul 27 '11 at 8:26
14

In both of the examples in which that is optional, the relative pronoun is the object of the embedded clause.

Long books [that] religious people like tend to be Bibles. [Religious people like long books.]

Water tanks [that] fish need are spacious. [Fish need water tanks.]

This is also allowed when the relative pronoun is the object of a preposition or another oblique argument of the embedded clause:

This is the boat I escaped in. [I escaped in this boat.]

In your other examples, the relative pronoun is the subject of the embedded clause:

Those that are rotten must be thrown away. [Those are rotten.]

Cars that break down endanger pedestrians. [Cars break down.]

English only allows you to omit that when it has been moved from a non-subject position in the embedded clause, and when it's followed by the subject of the embedded clause. I suspect that the reason for this is the ease of comprehension on the part of the listener. A sentence like Cars break down endanger pedestrians, if it were grammatical, would be extremely hard to parse.

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    Relative clause "that" can be omitted before the subject of a relative clause. (It doesn't have to represent the object of the relative clause, though it may -- "This is the boat (that) I escaped in".)
    – Greg Lee
    Sep 21 '16 at 18:08
  • @GregLee that's an important clarification. I'll updated the answer. Sep 21 '16 at 18:30
  • "Bulldogs bulldogs bulldogs fight fight fight.”
    – Barmar
    Dec 12 '19 at 5:20

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