I wouldn´t worry - both are correct. It´s like what is the verb from ´disliked´? You could either say ´to like´ or ´to dislike´.

Unlike the English language there is an infinitive of the negative of the verb, but it´s still a conjugation, as -me/ma are suffixes.

I don´t think you can simply say sevmek and sevmemek are two completely different words with the opposite meaning. They are both based on the same verb root ´sev´, only sevmemek is the negative of it.

But as sevmemek is an infinitive, I think it´s a bit harsch that you were corrected. They should have made it more explicit that they wanted to hear the original verb root.

Yes, exactly what I meant. I didn´t say they were different words with different word roots, but just that they have opposite meanings, and if their dictionary states it as a verb, they should accept is as a correct answer.

However, the most important thing for me is that I understand a certain grammar rule.