2

We are not sure if he is coming to the party

We are not sure that he is coming to the party

Which one is grammatically correct and why?

  • 2
    leaving this as a comment rather than an answer because I don't have an answer to your more important question, which is "why": in spoken English these phrases are both used and synonymous. The first sentence is usually, but not always, ended in "or not." The second sentence cannot end in "or not." In my own usage, the second, but not the first, is in formal writing. But I don't know if that's just me or if that's a "rule." – hunter Dec 15 '13 at 19:27
  • I agree that the two versions are very close. A subtle/subconscious distinction may be, perhaps, if is used in the case that the potential party-goer is the one making the choice whether to attend; and that is used when the intent to attend was there but circumstances have put the arrival in doubt. – toandfro Dec 15 '13 at 20:52
  • Both of these are grammatically correct, but some odd things happen when you alter the circumstances. For example, one can say, "We are sure that he is coming to the party," but you would not say, "We are sure if he is coming to the party." (Of course, under those conditions, you need not use either word; "We are sure he is coming to the party" is fine, too – perhaps even preferable.) – J.R. Dec 15 '13 at 23:58
2

If you are addressing a question of whether ‘he’ will or will not come to the party, you are more likely to use the if version:

Is Bob going to be there?
We’re not sure if he’s coming; he may have to be in New York next week.

The that version is more likely if you are correcting an assertion or assumption that ‘he’ will come to the party:

I can give Bob the news at the party tonight.
Well, we’re not sure that he’s coming; he left work early because he felt sick.

Note that the sentence is perfectly grammatical with no conjunction, which allows it to cover both uses:

We’re not sure he’s coming to the party.

0

As a standalone statement, the first will be more natural - not that the second is wrong, just this phrasing of "binary outcome" is more common.

We are not sure if he is coming to the party or not.

The other one would be found with more alternatives.

"We are not sure that he is coming to the party, but he will likely meet Joan that evening anyway, and possibly he may help organizing the party too, or stay the first hour of it or so."

You'll use "if" for the simple alternative "...or not". You'll use "that" for a selection: the party being one of multiple options.

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