Okay, I searched them up. Morpheme are words that cannot be broken down further. For example, the morpheme of the word "Incoming" is "in", "come", and "ing".
Allomorphs are any versions of a morpheme.
Accusative cases (as the ones talking about in this exercise) are objects that the verb is describing. Its like something specific. So you could say "Read the newspaper". In normal meaning, people will think you mean "read any newspaper".
In accusative case, your meaning of the sentence will become "read that newspaper SPECIFICALLY (for example read the newspaper that is over there) "
I hoped this helped you :)