Duolingo should allow a learner to proceed by skipping the item for the remainder of the lesson in a case like this where there seems to be an error, the learner has flagged that, and the learner would by lying if they were to write down something that they did not hear. Having no other option, I am about to enter "Úsáidimid an euro", but what I actually heard was "Úsáidim an euro".
Yes and yes (there are only úsáid and athúsáid ). The best way to determine which conjugation a verb uses is to look at its third-person future form. Since the third-person future form for úsáid is úsáidfidh, it’s a first conjugation verb. (Endings of -faidh and -fidh are first conjugation; endings of -óidh and -eoidh are second conjugation.)
You asked if there were a way, and I responded with a way. (It’s analogous to Latin, where one needs to know the first person singular present indicative and the infinitive to know which conjugation a verb belongs to.)
You don’t need to know how a verb conjugates to find out which conjugation it uses — you could refer to a site like this one. Only you can determine whether that’s a help for you or not.
Sorry, I don't mean to sound ungrateful. It just seemed a bit like circular logic. I know I can check a verb in the dictionary, but I know for example that a regular verb with a two-syllable root ending in 'igh' will conjugate a certain way without having to look it up and I suppose that's the sort of thing I was hoping for. Irish has so few irregular verbs, after all. But I do appreciate your help.