I recently was corrected in this SO answer. I am just curious if there are any differences between the following phrasings, and whether there is a difference in meaning or one phrasing simply sounds wrong:

Not sure does it work


Not sure if it works

As far as I know the second is subjunctive and the first is not, but both seem to be correct to me.

3 Answers 3


Well, neither one is a complete sentence, but you probably realize that. Let's go ahead and make them complete sentences anyway:

  1. I am not sure does it work.

  2. I am not sure if it works.

The only correct sentence is 2.

In 1, "does it work" is a complete question, also known as an interrogative clause. Notice the inversion of "it" and "does," which are the subject and the modal auxiliary verb, respectively. This is a sure sign of a question. In this form, it needs to stand by itself: "Does it work?" Rearranged into a regular declarative statement, it would look like this: "It does work." So "does" is just part of the verb phrase. It doesn't take on any conjunction-like duties.

In other words, not only is there no subordinating conjunction such as "if" to join "does it work" to the main, independent clause, but interrogative clauses can't be subordinated (although they can have subordinate clauses of their own, as in, "Are you sure that it works?").

(The subjunctive mood is not present in either 1 or 2.)

  • I think I've got that, speaking with a programming terms the listener expects a statement after I am not sure, and I was giving him a question. So would it be grammatically correct to say I'm not sure if it does work? Commented Jun 20, 2015 at 3:56
  • It would be grammatical although "...it works" is probably more natural than "...it does work". The auxiliary "does" is kind of used for emphasis and would be difficult to explain completely.
    – shawnt00
    Commented Jun 20, 2015 at 4:36
  • @shawnt00 yes, I understand such form will require some counterpart against which the emphasis was done, such as some noise of background info. Commented Jun 20, 2015 at 5:03

"Does" is a verb (albeit an extremely flexible one). "If" is a conjunction. They're not interchangeable (although sometimes you can construct sentences that are grammatically distinct but use identical sequences of words aside from "does" and "if").

The mood is irrelevant, although the second is actually indicative, not subjunctive: it's expressing uncertainty about the functionality of something. The first example is not grammatical, although with a bit of punctuation you could make it say something similar:

Not sure; does it work?

This is, however, a question that implies doubt, not a statement of doubt directly.

  • I assume, not sure; does it work? can be said only in response to a certain proposal & idea, i.e. another person is an initiator. This structure cannot be used if the speaker himself proposes something, right?
    – user18856
    Commented Jun 20, 2015 at 8:08
  • @AmD: Yeah, the context is different. Commented Jun 20, 2015 at 11:05

The first one really doesn't make natural sense to an English speaker.

Placing "does" at the beginning of the clause, as in "does it work", turns it into a question but yours is clearly not part of a question.

You were looking for something along these lines:

[I am] Not sure that it does work in ruby 2.0...

[I am] Not sure that it works in ruby 2.0...

The omitted "I am" would be clear in casual conversation but you wouldn't write this way in a formal setting.

Some people would argue that it's more correct to use the word "whether" rather than "if". It isn't really part of a conditional/subjunctive as you mention, it's really more about a list of choices. In this case the choices are "it works" and "it doesn't work".

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