So am I right assuming that aspects are used differently depending on whether the sentence is affirmative or negative?
Because we have "Nesmíš jíst tak rychle!" and "Smím sníst tu poslední hrušku?". Or am I missing something else?
Your assumption is incorrect.
I don't know what to add to JanLyko's answer above. Perhaps try googling what 'perfective' and 'imperfective' actually means?
I am sorry if the way I worded my question was ambiguous but I was not speaking in terms of general use of aspects but referring to the use of perfective/imperfective infinitives following negative/affirmative modal verbs.
I just had a look at online resources I found here https://forum.duolingo.com/comment/24326912 but I do not really come to a conclusion.
So I'm still wondering why it is "Nesmíš jíst" but "Smím sníst". Maybe you could explain this specific example?
Writing this post, I just realized it might about the first sentence being a statement of general truth while the second sentence is situational. But still I would appreciate some clarification.