"The teacher fights with the parent."
Translation:Troideann an múinteoir leis an tuismitheoir.
In standard Irish, as well as in western and southern dialects, preposition + singular article is the environment for eclipsis, not lenition . So, according to the rule, it should be 'leis an dtuismitheoir'.
But this does not happen here, because d and t are not eclipsed in this case. If I am not mistaken, only Munster dialect is occasionally an exception to this.
For some issues getting it totally wrong is still accepted as right as the difference is small enough: "na" could be a typo of "an" and adding an "í" at the end of "thuismitheoirí" is again just one letter off, which makes this acceptable as well.
So, no, you were not correct, DL just closed both eyes ;-)
No doubt we all compensate that with omitting a "the" every now and then, where the translation is basically correct, but not from a linguistic point of view 100 % on target.
When le comes before a definite article (either an or na), it becomes leis. This leis is NOT the prepositional pronoun leis.
Troideann sé le Pól - "he fights with Paul"
Troideann sé liom - "he fights with me"
Troideann sé leat - "he fights with you"
Troideann sé leis - "he fights with him"
Troideann sé léi - "he fights with her"
Troideann sé leis an tuismitheoir - "he fights with the parent"
Troideann sé leis na tuismitheoirí - "he fights with the parents"
Troideann sé lena thuismitheoir - "he fights with his parent"
Troideann sé lena tuismitheoir - "he fights with her parent"