Generally speaking, you should not end a sentence with a contraction of a verb and a noun or pronoun.
(I'm, you're, he's, she's, it's, we're, they're, I've, you've, we've, they've, I'd, he'll, etc.)
You cannot say:
Do you know who I'm?
Instead you say things such as:
Do you know who I am?
Do you know how many kids John has?
Can you tell me where they are?
Yes, I will.
Especially in spoken English, you'll hear sentences such as these ending with a contraction of have and another verb and/or the word not:
=> Now that you mention it, I think he must've. (...he must have done that)
=> You shouldn't've!
It's OK to end a sentence with a contraction of a verb and the word not:
=> No, he isn't.
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