When the word that is used in a sentence to introduce some relative clause it is always an essential element which follows. Therefore, no comma is required. Example:

I'm sure that you are lying.

When I leave out the word that, it is still a correct sentence, right? Do I have to use a comma then? Because, when you read the sentence aloud, it kind of feels like there is a pause. Examples

I'm sure you are lying.

I have the evidence right here in my pocket (that) you are lying.

Especially in the second case, I would prefer a comma at the place of the that. Is there a strict rule?

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    The use of commas in written English sentences is always governed by the intonation of the spoken sentence. It is not governed by grammar, and certainly not by whether or not an optional pronoun is retained or deleted. Don't worry about commas. – John Lawler Sep 16 '13 at 16:42
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    @JohnLawler Would that it were so! In my experience the use of commas is usually governed by the arbitrary prejudices of the publisher--occasionally by the arbitrary prejudices of the author. – StoneyB on hiatus Sep 16 '13 at 17:56

The use of commas to set off relative clauses has a specific purpose. In general, they are used to denote clauses that are either parenthetical or otherwise nonrestrictive.

In your examples, the clauses are restrictive, that is, they are essential to the meaning of the sentence and specifically delimit the phrase they modify. As such, the addition of commas would convert them to nonrestrictive and diminish the import of the content of the clause in relation to the modified phrase.

While the omission of that changes the flow a bit, it should not be replaced with commas lest the reader think the clause that follows is not critical to the main thought. In fact, I think that the omission of the relative pronoun is common for restrictive clauses, but not for nonrestrictive clauses (I welcome others weighing in on this).

The last example might flow better if it were reworded slightly

I have the evidence (that) you are lying right here in my pocket.


In the first example, there is really no question: you cannot put a comma there.

According to APA (6th ed., p. 88), in these situations commas should only be used

to set off a nonessential or nonrestrictive clause, that is, a clause that embellishes a sentence but if removed would leave the grammatical structure and meaning of sentence intact.

Adding a comma implies that the clause is not critical to the meaning of the sentence.

The second example might be better if reworded like this:

I have the evidence that you are lying right here in my pocket.

Separating the adjective clause from the noun it modifies is generally a bad idea.

If you want a pause in it, the meaning of the sentence is slightly different, but you could do that—not, however, with a comma. A semicolon, a colon, or even just a period is acceptable there. If you do this, they become two independent clauses, and one cannot separate two independent clauses with a comma; that's a comma splice.

I have the evidence right here in my pocket; you are lying.


I'm sure you're lying is fine without a comma. I have the evidence right here in my pocket, you're lying, on the other hand, isn't fine. It does need that or it needs a semi colon, not a comma.

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