I know that some verbs take interrogative clauses ("I know where they are") while others don't (* "I believe where they are"). The verb "sure" is kind of like "believe", but unlike "believe" it seems to allow interrogatives under some circumstances, such as under negation:

*I am sure whether it's raining.

I am not sure whether it's raining.

To be clear, this is about interrogative clauses in general, not just 'whether' ones. Consider also:

I am not sure why the floor is wet.

*I am sure why the floor is wet.

I am not sure why this is so. Are there any explanations of this?

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    Do you sure it is a verb? ;)
    – J-mster
    Jun 16, 2020 at 4:53
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    Try this on English Language & Usage Jun 16, 2020 at 9:29
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    "I have a strong belief about the current precipitation status" Jun 16, 2020 at 13:03
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    @HagenvonEitzen I'm not sure what you're trying to say with your comment. I don't doubt that "I'm sure whether it's raining" can be paraphrased into an acceptable sentence. The question is why it's not acceptable as it is.
    – eyet
    Jun 16, 2020 at 16:11
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    Is it really wrong? It's a useless thing to say (because you could just say whether it's raining or not) but I don't see why it's wrong or not allowed. Of course, I am not a linguist (hello, you're on HNQ).
    – user253751
    Jun 16, 2020 at 17:31

6 Answers 6


(Be) sure (not sure of or sure about) is a predicate that doesn't allow Wh-clause complements in statements, though it can take that-clause complements:

  • *I'm sure what he's doing.
  • I'm sure (that) he's doing something.
  • *Bill was sure when he was leaving. ( While he was leaving, he was sure)
  • Bill was sure (that) he was leaving soon.
  • *She's sure whether she's leaving.
  • She's sure that she's leaving.

Interestingly, this is not prohibited with negative uses of sure,

  • I'm not sure what he's doing.
  • Bill wasn't sure when he was leaving.
  • She isn't sure whether she's leaving.

including questions (which trigger negative polarity items)

  • Are you sure what he's doing?
  • Was Bill sure when he was leaving?
  • Is she sure whether she's leaving?

This appears to be because not be sure means not know idiomatically, just like Are you sure? means Do you know?. Like know, in this construction, sure can take a Wh-complement. NPIs interacting idiomatically with syntactic constructions are not uncommon in English.

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    Great, thank you. This looks like the right explanation to me. Could you elaborate a bit on your last paragraph, especially the "NPIs interacting idiomatically with syntactic constructions" part?
    – eyet
    Jun 16, 2020 at 17:06
  • The negative sense of sure has an idiomatic sense that allows it an affordance (taking a Wh-complement) that it does not have if it's not negative. The affordance is a one-off copy of the same affordance for know. At least that's how I'd attribute it. This kind of stuff happens all the time with idioms, and few things are more idiomatic than NPIs.
    – jlawler
    Jun 16, 2020 at 17:11
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    This... isn't entirely true. The sentence "I am sure whether it is raining" has clear semantics (I am claiming to know a fact that I am not specifying), and is immediately comprehensible in exactly the same manner as "I don't know; can you go to the bathroom?" Jun 17, 2020 at 11:24
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    @chrylis-cautiouslyoptimistic- The question is not whether it has clear semantics or not; the question is why it sounds weird, in a way that other phrases do not.
    – eyet
    Jun 17, 2020 at 15:32

Words like "whether" in English are sometimes called "Negative Polarity Items" - because they can occur in sentences that are the negative version of a sentence they can't occur in. In many varieties of English, "any" and "ever" are also NPIs, but not in every variety. Compare "*we have any bananas" to "we don't have any bananas" or "*I ever ate a banana" to "I didn't ever eat a banana."

NPIs are frequently allowed in other situations (sometimes referred to with jargon they are licensed in other environments) - specifically ones referred to as "downward entailing environments." Downward entailing means "if you get more specific, it is still true" - and think about it, "I didn't eat a fruit" entails the more specific ('downward') "I didn't eat a banana" - while the positive version, "I ate a fruit" does not downward entail the more specific "I ate a banana." Other downward entailing environments are questions ("did you know whether she ate a banana?") and conditionals ("If she would ever eat a banana, she'd find it delicious". A more thorough and more humorous explination is https://allthingslinguistic.com/post/113907456893/a-detailed-explanation-of-negative-polarity-items

If you want a video that explains this, http://www.thelingspace.com/episode-74

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    Correct use of italics instead of " " would make your answers more readable. Jun 16, 2020 at 7:48
  • "We have any bananas" is grammatically correct, when written as a question, because there would be an implied "do" at the beginning of it.
    – nick012000
    Jun 16, 2020 at 13:28
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    @nick012000 Or, if you have a large stock of bananas, and someone asks about a particular type of them: "Do you have Lady Finger bananas or Lakatan bananas?" "We have any bananas" Jun 16, 2020 at 14:25
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    Are other wh-words also negative polarity items? Otherwise this doesn't really explain the issue. You can say "I'm not sure which cup is mine", but not "I'm sure which cup is mine", and "I'm not sure how you did that", but not "I'm sure how you did that".
    – eyet
    Jun 16, 2020 at 15:49
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    The NPI isn't the Wh-word; it's using a Wh-complement with (be) sure. Similar to adverb-fronting with Subject-Auxiliary inversion: Never/Rarely/*Frequently/*Occasionally/*Yesterday have I seen anything like it.
    – jlawler
    Jun 16, 2020 at 18:35

"Whether" implies that there are two options.

"I am not sure whether it's raining" includes the implication that it may not be raining. One could also say "I am not sure whether it's raining or not", or "I'm not sure whether it's raining or snowing"

"I am sure it's raining", on the other hand, means that it is definitely raining. There is no implication that something else could be happening instead.

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    "I am sure whether it's raining or not" would be grammatically correct, though. It's saying you know what state the weather is in (either raining or not raining), without specifying which one is true.
    – nick012000
    Jun 16, 2020 at 13:30
  • This doesn't seem to explain the issue. The question is why you can't say "I am sure whether it's raining", not "I'm sure it's raining".
    – eyet
    Jun 16, 2020 at 15:45
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    This is the closest to the correct answer. The last missing piece is that OP's phrase is an implicit contraction - "I am not sure whether it's raining" is not technically correct English on its own, it's an implicit contraction of "I am not sure whether it's raining [or not]". As you note, the use of 'whether' requires that the speaker is presenting more than one option.
    – J...
    Jun 17, 2020 at 15:12
  • @J... This isn't a good answer because the question is not about 'whether' specifically. The question is about interrogative clauses (what, when, why, how, etc.). This answer does not generalize to those cases.
    – eyet
    Jun 17, 2020 at 15:33
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    @eyet Yes, but OP's 'whether' example is a special case - there's no generalizing that exception because it really isn't one (the missing piece I alluded to in my previous comment). So, yes, I agree the answer is incomplete without that missing piece, but it's the closest answer here to actually getting at the source of the confusion.
    – J...
    Jun 17, 2020 at 15:37

Example: "I'm sure that it's raining, but we will go out tomorrow whether it's raining or not."

"Whether" envisions possibilities of roughly equal epistemic status (truth value), producing uncertainty. It is ungrammatical when asserting certainty, which is epressed by "that", or less emphatically without any preposition ("I'm sure it's raining").

Example: "I'm sure that it's raining, but we're going out whether it's raining or not."

Here, it is raining in the real world, but I also envision an abstract world in which it may or may not rain, but I drag you out regardless. Get your coat on!

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    This is not the kind of use of "whether" I am asking about. I am asking about "sure whether X" not "sure that X", and I'm asking about interrogative clauses in general. You can say "I'm not sure where they are", but not "I'm sure where they are".
    – eyet
    Jun 16, 2020 at 15:43
  • I don't think these are different usages. "I'm sure whether" is ungrammatical because I'm expressing certainty, not uncertainty. Jun 20, 2020 at 13:48

Beginning a conversational topic with a confession of ignorance about something is not a normal thing to do unless you want to know about it. So the confession is conventionally taken as asking a question. Embedded questions are introduced with "whether", and that's why the "whether" in your example crops up after "I'm not sure" but not after "I'm sure".

  • 1. The phrase "I am not sure whether ..." doesn't have to begin a conversational topic. It can also be an answer to a question. 2. Your explanation doesn't generalize because you can say "I know whether X" in a way that you can't say "I am sure whether X".
    – eyet
    Jun 17, 2020 at 15:31
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    @eyet But if someone asks you whether he left, and you reply "I don't know", you have not thereby asked a question.
    – Greg Lee
    Jun 18, 2020 at 2:25
  • Yes, of course not. And neither do you ask a question by answering "I'm not sure", so that seems to contradict your answer.
    – eyet
    Jun 18, 2020 at 4:37

The term "whether" in a sentence like this is usually a modified use of the phrase "whether or not", which begs the question of whether (or not) we are implying a double negative, as in "I am not sure whether (or not) it is raining", which would be misleading because it seems to be saying "I am sure that it may or may not be raining".

It therefore seems likely to me that the "not", from "whether or not", is simply being transposed to the beginning part of the sentence structure, which is the answer to your question as to why one is more appropriate than the other: because the "not" should appear only once, in the beginning or ending part of the sentence.

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