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"I don't like him because he is smart."

This sentence could mean (ignoring the context) both ...

1. The reason I don't like him is his smartness.

2. The reason I like him is NOT his smartness but something else.

Then, could this sentense mean the blow?

3. The reason I don't like him is NOT his smartness but something else.

Thanks.
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1 - yes
2 - yes
3 - no

If it's #1, then you are telling me the reason for your not liking him. You're not saying you don't like him but it's for some other reason. That would be something like "I don't like him but not it's not because he's smart."
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handtalk3. The reason I don't like him is NOT his smartness but something else.
No. You have only one negation in the original sentence. It can apply to the liking or to the reason, but not to both!

CJ
Thank you.
I see.

Thank you.
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