EN: not sure + if / whether / that


What would you put after "I'm not sure": "if" or "whether"? For instance, would you say: "I'm not sure if there is another stop" or "I'm not sure whether there is another stop"?
Thanks for the help!
  • Pierre Simon

    Senior Member
    Hullo ParisLondon

    You can use 'whether' when there are two possibilities, and you don't know which one to choose.

    'I don't know whether or not there is another stop'
    'I don't know whether there is another stop or not.'

    Alternatively, 'I don't know if...' is fine.

    Pierre Simon

    Senior Member
    Or indeed more than two.
    Quite right, or indeed more than two. No excuse, except that post #3 was 'typed' on a 'phone, in a hurry.

    'I'm not sure whether to get the Piccadilly Line and change at Oxford Circus (or get the Northern Line, or get the bus, or walk, or give up and go back home).

    The myriad possibilities are, of course, generally left unexpressed, or simply 'bundled up' in the two words 'or not'. For the sake of completeness, it may be worth adding one further point about the difference between 'I'm not sure if...' and 'I'm not sure whether...'. If the doubt being expressed concerns an action to be taken, rather than a state of affairs, then you can use use 'whether' followed by an infinitive.

    'She is not sure whether to leave this afternoon or tomorrow morning.'

    This kind of construction does not work with 'if'. You would have to insert another verb :

    'She is not sure if she should leave this afternoon or...'


    Senior Member
    English Canada
    Personally, if the speaker does not have an alternative in mind, I would simply use that.


    Senior Member
    Spanish, Spain
    I have a similar doubt. Let's take the following sentence:

    "The coffee is bitter. I'm not sure whether to add some sugar (or not)".

    Is "or not" compulsory in that sentence?
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