Translation:He fights with the man but why?
Could someone tell me why it is somethings "le ..." and sometimes "leis ..." pelase?
Here "with the man = leis an bhfear", but a few sentences ago we had "with my father = le m'athair", and I'm not sure why.
"leis an bhfear" certainly sounds more correct than simple "le", but neither of "le m'athair" or "leis m'athair" sound any more correct than the other.
The leis in leis an bhfear is not a prepositional pronoun, it's just a preposition (meaning "with").
The leis in troideann sé leis. (with no following definite noun) is a prepositional pronoun (meaning "with him").
Compare "he fights with her" - troideann sé léi, "he fights with the girl" - troideann sé leis an gcailín.
Just to show that English isn't immune to this kind of confusion, compare
"he fights with him" - "he fights with his brother"
"he fights with her" - "he fights with her brother"
note how English uses "him" for the pronoun and "his" for the possessive adjective, but uses the same word "her" for both the pronoun and the possessive adjective.
After reading all the arguments back and forth about this, it brings to mind a funny moment talking with my long-ago organic chemistry professor. Students would fight with him over the name of some gigantic branched molecule. They would insist on being right. He said, it is your job to observe the universe as it is. Stop trying to change it.
In English, there is a difference between "he fights" and "he is fighting". The first one means that he often fights, he fights habitually--for instance, "he fights on Thursdays" or "he fights when he gets angry". The second means that he is fighting right now.
In Irish, these two different forms exist as well. Troideann sé translates to 'he fights'. If you wanted to say 'he is fighting', I believe you would say: 'tá sé ag troid'.
Cen fáth a bhfuil tú ag troid leis/léi?
In Munster Irish it would probably be go bhfuil rather than a bhfuil - cén fáth is already a question, you don't need an interrogative particle before the verb:
Cen fáth go bhfuil tú ag troid leis/léi?
You use the verbal noun after ag for the progressive verb ("-ing"). The verbal noun doesn't have any tense, as the tense is indicated by the verb "be"/bí - "why were you ...?", "why are you ...?" and "why will you be ...?" in English, and a raibh, a bhfuil and a mbeidh