What is the difference between mange and manges?
It doesn't seem to be explained in the lesson and I can't see the difference between when each one is being used.
They don't really ever explain it, but that's called "conjugating" a verb. Basically there are different versions of each verb, depending on things like the subject (I, you, they, he/she, as nbeckman said), and sometimes the tense (past, present, future). If you go to the vocabulary tab at the top you can find a list of all the conjugations for each verb they've taught you so far, though it would be nice if they had a lesson specifically on conjugation rules and stuff.