When you want to combine "I think" with "he cannot do it", usually it goes like "I don't think he can do it"--not like "I think he cannot do it". Then, what about "I think" + "he shouldn't do it"? The same pattern, like "I don't think he should do it"?
) So it is not a matter of the modal, or of any particular verb in the subordinate clause, but a matter of the verb in the main clause. The meanings are for all practical purposes equivalent between "I don't think he can do it" and "I think he cannot do it".
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MM, could you please paraphrase this part of your comments? I don't think I really understand it:The meanings are for all practical purposes equivalent between "I don't think he can do it" and "I think he cannot do it".
This is called 'transferred negation', Taka, and is common with verbs of opinion (think, believe, appear, seem, expect, etc.) So it is not a matter of the modal, or of any particular verb in the subordinate clause, but a matter of the verb in the main clause.
"I think + negative or positive" is used as a response to negate or strengthen a prior statement.
1) I don't think he can do it. VS I think he can't do it.
2) I don't think he should do it. VS I think he shouldn't do it.
I suggest that the second ones in each sentence are much stronger and they do not represent the normal neutral fashion as represente