Cannot: Why one word? (can't vs. cannot vs. can not)

[Beeesneees]

Although I also agree with Logical's usage of 'can not', I dfon't think you have understood it in the same way judging from your last example. I would use 'cannot' in that final sentence.

The only time I would use 'can not' would be in the same way as Logical describes - to indicate choice.

I can go to the river, or I can not go (ie I can choose not to go) there - it is up to me.
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Cheers m' dears!

[Dbinaz]

I know this is an old thread, but I need to clear something up that is similar. I am developing a web application and I need advice on the use of the following. Which is the most correct option?
It is a validation message for a sign up form.

1. Email can't be empty/blank
2. Email cannot be empty/blank
3. Email must not be empty/blank

Thanks,
Derek

[Beeesneees]

I would use 2 or 3 for the application. 'Can't' seems a little too informal, even though you may wish to appear friendly.
I think I would prefer 'blank' to 'empty'.
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Cheers m' dears!

[Dbinaz]

I think I like 3 the best, with 'blank', as it sounds more professional. Thanks!!