The words with negative affixes but have positive meaning.

Hello, guys. Today I had an English test, and got a wrong answer about the words with affixes. I thought 'unloosen' and 'loosen' were antonyms, but they were synonyms. Honestly, when I found out that they were synonyms, I wanted to ask what the point of putting affixe is, then? ?_? Well, I heard there are lots of words like those two which attaching affixes doesn't make meaning difference to . So, if you know some of these words, please tell me. Thank you so much!!!