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I currently can't think of a good reason grammatically; but there are plenty in terms of clear communication.

marked as duplicate by Edwin Ashworth, Robusto, tchrist, FumbleFingers, Sven Yargs Sep 8 '15 at 4:42

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No, it's not correct. I'm assuming a conversation like:

A : Are they working on the project?
B : I'm sure they are.

"Are" is an auxiliary verb. You're not saying they are (as in, they exist), you are saying they are working.

Because of the conversational structure, you don't need to repeat "working" as it is apparent since you're immediately answering a question.

However, you stress the "are" when you reply. Therefore, you can't contract it because you fully pronounce it.

B : I'm sure they are
(you stress the last word)

If you want to contract the "are", you need to add "working" to the sentence.

B : I'm sure they're working
(stressing the last word again)

Note It does sound weird if you repeat the "working". It's correct, but less fluent than saying "they are".

Note While you cannot contract the verb, you can still contract the negation:

B : I'm sure they aren't
(stress on the bold letters)

Afterthought Come to think of it, when you negate, you can actually contract the verb, as you are now stressing the negation instead of the verb.

B : I'm sure they're not
(stress on the bold letters)

  • IF you can pronounce/verbalize it, you can write it. Such short forms is not so much as grammatical but merely reflection of the actual elision of phonemes. – Blessed Geek Sep 7 '15 at 15:55

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