"He fights with the man but why?"
Translation:Troideann sé leis an bhfear ach cén fáth?
There are two different words spelt "leis." The first is the combined form of "le + sé", meaning "with him." But there's another transformation some prepositions--mainly those that end in vowels--undergo when they're followed by "an":
i + an -> sa faoi + an -> faoin do + an -> don de + an -> den
le also does a transformation like this, but it keeps the "an":
le + an -> leis an
I'm not sure why it ended up like that instead of, say, "len" or "lén", but that's just the way it is in Irish. Something to memorize and be careful about, since it's so easily confusable with the le + sé leis.
Ulster Irish uses a séimhiú rather than an urú after "preposition + an", so they say leis an fhear in Donegal Irish.
Even though Duolingo teaches ag an bhfear, a decision was made to accept ag an fhear as an acceptable answer in exercises like this, for the sake of people who had already learned some Ulster Irish. As there is no way for Duolingo to tell whether you entered ag an fhear because you are learning Ulster Irish, or because you couldn't remember what you had been taught here on Duolingo, Duolingo doesn't actually know if it was letting you away with it, or if your answer was actually correct.
Thanks very much, that's really interesting! I actually did grow up in a border county and I've noticed some of the things I learned in school are a bit different from what Duolingo teaches, especially with pronouncing the ends of words (like saying the 'a' in 'nua', or the 'u' in 'inniu'). I think it was just a mistake this time but I'm future if I confuse my urú and séimhiú I'll be sure to blame it on Ulster Irish.