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Is it true that we should not write positive then negative sentences?
e.g. I think he cannot remember the exact date.
I know he is not a nice guy.

Thank you.
Comments  
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It is not true, but native speakers tend to negate the main clause particularly with the verb 'think' and its relatives:

'I don't think he can remember' is more natural than 'I think he can't remember'.
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Thank you very much, Mister Micawber.
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Hi Hongkie,
I'm not sure what you mean.
Do you mean positive as in good, or simply as in not with a "not."
Is it true that we should not write positive then negative sentences?
e.g. I think he cannot remember the exact date.
I know he is not a nice guy.
These are not examples of a positive and then a negative sentence. These are two clauses of the same sentence.

I think | he cannot remember the exact date.

What you heard about this is not true in the general case. However, in the case of think, the negation is normally in the first clause even though it applies to the second clause. To express the thought in the sentence above, use:

I don't think | he can remember the exact date.
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Similarly:

Susan doesn't think John wants to go to the movies tonight.


means

Susan thinks John doesn't want to go to the movies tonight.

But it's said as shown in the first sentence of the two.
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Your sentence I know he is not a nice guy should not be changed. It's fine as is.

CJ
To me, a negative sentence is a sentence with a negative word like "no", "not", etc.
A positive sentence is a sentence without any negative word.
Students: We have free audio pronunciation exercises.
Thank you, CJ.

I know I have problems with concepts.

Can you please give me an example of a positive then negative sentence.

Is "I have no money" a positive then negative sentence?