1. You should do it yourself.

2. You should do it itself.

Are both grammatical?

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3. She must be angry because she's not saying anything.

4. She has to be angry because she's not saying anything.

Are both grammatical and commonly used?

4 answers

1 is fine, but 2 doesn't make sense.

3 and 4 are pretty much the same.
Thank you for your help.

5. He hit the boy himself.
6. You should eat the apple itself.

Are the sentences grammatical?
7. I like the boy themselves.

What about this sentence?
5 means that the subject hit the boy; no one else hit the boy.

6 doesn't make sense; remove "itself."

7 doesn't make sense either; remove "themselves."