I'm not sure if(or, whether) we can do it.

Can this sentence be written in this way...

I'm not sure can we do it.

Can whether, or if, be omitted like that and still be understandable?


  • You could omit either or both of whetheror if, but you would need to say I'm not sure we can do it.
    – WS2
    Dec 30, 2014 at 14:54
  • 1
    Did you mean to include the inversion to "can we" or was that a typo with only the omission intended?
    – Jon Hanna
    Dec 30, 2014 at 15:20
  • These Google Ngrams seem to indicate that omitting the complementiser is the most common practice, with using 'that' the next most common, for I'm not sure ___ can we do it. Dec 30, 2014 at 15:25
  • @Jon I've once seen that the inversion is needed to omit if. That is why I wrote that way.
    – hjjg200
    Dec 30, 2014 at 15:26
  • 1
    You can, but it won't mean exactly the same thing. Dec 30, 2014 at 16:08

4 Answers 4


Consider first this question on omitting "that" from a sentence. As the answer says, we can omit it when it is used as a subordinating conjugation, though there are some cases where it remains either necessary or clearer (this article has more advice on that).

Now, having considered that, consider that when we change from:

She told me that it was okay.


She told me it was okay.

In the result we don't really have an omitted that. What we have is an independent and a dependent clause conjoined together by nothing. Granted in this case that is the only English word that could do the job, but that doesn't really make any difference; if English had a million such words, or if someone's idiolect meant they often used que there, the result would be the same.

So really the phenomenon isn't that we can omit "that" at all, but that we can omit subordinating conjunctions.

In the examples you give here, if and whether are being used as subordinating conjunctions. And so they can be omitted similarly:

I'm not sure we can do it.

Now, we can't say looking at that whether it was if, whether or that which was omitted, again because it wasn't really any of them; we have subordinating conjunction happening without a word doing the task. The meaning is the same either way (and if not, that's a clear sign that you don't have a case where the omission is safe). Dropping if and whether here follows the same restrictions as mentioned in the two links above for that.

Inversion does something completely different:

We can do it. — Indicative statement.

Can we do it? — Question formed by inverting the modal verb in the indicative.

This doesn't mix in with the omission of if or whether because it doesn't work without the omission:

*I don't know if can we do it?

You could combine the two clauses with a comma or full stop, but then it's something different again:

I don't know. Can we do it?

This is not an omission of the subordinating conjunction; it is making the two clauses more fully separate so while it arrives at something grammatical, the meaning is completely different.

  • Hmm, not sure you can freely omit whether or that. Consider I asked I could go for example. :) Dec 30, 2014 at 15:52
  • @Araucaria that's covered in the links included that explain "there are some cases where it remains either necessary or clearer" and the second that gives advice on precisely that.
    – Jon Hanna
    Dec 30, 2014 at 16:02
  1. I'm not sure if we can do it.

  2. I'm not sure whether we can do it.

  3. I'm not sure we can do it.

Sentences (1) and (2) both include a SUBORDINATOR which marks the following clause as subordinate - and also, in these cases, as interrogative. These subordinators are if and whether. We can think of them as the interrogative counterparts of the subordinator that. The subordinator that is used to mark declarative subordinate clauses:

  • That my elephant is large is not disputed.

That is often omissible, and can easily be dropped in front of subordinate clauses in many instances:

  • I think that my elephant is fantastic.

  • I think my elephant is fantastic.

Both of the examples above are grammatical. We can't omit that however when the subordinate clause is the first clause in the sentence, so the following version of the large elephant example is ungrammatical:

  • *My elephant is large is not disputed.

In contrast, both if and whether are less easily dropped. Consider the following examples:

  • She asked if I was well.

  • She asked whether I was well.

  • *She asked I was well. (ungrammatical)

  • I wonder if she'll reply.

  • I wonder whether she'll reply.

  • *I wonder she'll reply. (ungrammatical)

The adjective sure can take either a declarative or interrogative content clause as complement. In other words we will see this phrase followed by clauses headed by either if or whether as well as by that.:

  • I'm not sure whether that's my baboon.
  • I'm not sure if that's my baboon.
  • I'm not sure that that's my baboon.

We have seen further above that we cannot freely omit if or whether from such examples, but that we can omit that. Example (3) above, therefore, would seem to be a version of:

  • I'm not sure that we can do it.

Although effectively this has a similar meaning to (1) and (2), it is not exactly the same. Nonetheless, we should test this theory and not take it for granted. After all, it's difficult to state exactly what the difference is here. We can test this theory by considering the same sentences, but with I am sure instead of I'm not sure. We will then easily see that the version without the subordinator seems to be equivalent to the version with that, not the versions with if or whether.

Let us suppose suppose that the following sentences are said in reply to the statement: You're not sure whether you can do it!:

  1. I am sure whether I can do it.
  2. I am sure if I can do it.
  3. I am sure that I can do it.
  4. I am sure I can do it.

Sentences (5) and (6) are equivalent. Whilst the speaker is saying that they know whether they can do whatever activity it refers to, they do not explicitly state whether they are able to or not. The next sentence in each case could well be And the answer's no, I can't. Alternatively the next sentence in these particular cases could equally be And I am going to do it.

In sentences (7) and (8), however, the speaker is clearly stating that they definitely can "do it". The sentences cannot be followed by And no, I can't.:

  • I'm sure that I can do it. And no, I can't. (nonsensical)
  • I'm sure I can do it. And no, I can't. (nonsensical)

This clearly demonstrates that (8), the sentence without the subordinator, is a version of (7) and not a vesion of (5) or (6). It also clearly shows that:

  • I'm not sure we can do it.

... is a version of:

  • I'm not sure that we can do it.

So the real answer to the Original Poster's question is that we can omit that from the sentence I'm not sure that we can do it, but we cannot omit if or whether from the original examples. Having said this, it does not make much difference which versions we use from the OP's examples as they effectively communicate the same thing.


I'm not sure we can do it, inserting whether or if as you please or omitting it completely. "I'm not sure whether/if we can do it."


As a matter of fact, your sentence sounds better if "whether/if" is omitted. (and no inversion):

I'm not sure we can do it.

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