"Nopțile sunt grele și urâte."
Translation:The nights are hard and ugly.
Why put "heavy" as an acceptable alternate when hovering over the word but not accepted in the answer
They are not, as you put it, 'acceptable' alternatives. They are alternative meanings to the words, but not necessarily in the context presented to you. You are given alternative forms for words to express different genders, pluralities, and pronouns; and have to choose the correct one. It is the same for when different meanings of a word are presented to you.
For two different concepts/meanings in one language there may be a word each to express them, where as in another language there may be just context, or you may have to use a combination of words to express the same meaning/concept.