When can the word 'that' be omitted?

Question

I've been thinking about it for some time and I just can't figure out the answer... Is it permissible to omit the word 'that' in some cases?

E.g. Which is more correct:

a). "One shouldn’t make a promise they can’t keep."

or

b). "One shouldn’t make a promise THAT they can’t keep."

I feel like the first one is wrong, but I hear it all the time so I'm confused...

Answer 1

I think this is the difference between formal language (used in writing) and informal or colloquial language used in speech. Of course there is also informal writing where leaving out the 'that' is acceptable. This is like informal or formal clothing - you have to judge what is best in any given situation, and if you get it wrong, then it looks wrong to others....

Answer 2

The second one is grammatically preferable, but the first is commonly used. Dr. Nightcall is correct when he points out the disagreement between one and they, but he has incorrectly inserted a comma after the word 'promise'.

Answer 3

I think both are correct but, like you, I prefer to include that. It sounds better, and is decidedly better English when it is written. Spoken language is not quite so formal so the first would be acceptable when spoken.

Answer 4

In addition to the "that" thing, both sentences have a subject/object disagreement, i.e., one/they.

One shouldn't make a promise, that one can't keep.

Answer 5

Hummmm, I like the first one

Answer 6

It may be omitted when the "that" is understood; as in your first example.