"My mother eats eggs for breakfast."
Translation:Moja mama je jajka na śniadanie.
why "eggs" is not in accusative in this sentence (what does she eat: eggs)
jajka [plural, Nominative] -> jajka [plural, Accusative]
Nominative and Accusative forms are the same.
Is 'Mama je jajka na śniadanie' also correct or would it not be clear whose mother is eating?
The English phrase states "My mother..." so it should be translated. You may say "Mama je jajka na śniadanie" in meaning "my mother" only if it is clear from the context whose mother is she.
when is 'dla' used rather than 'na'. i used dla in this sentence - why is it wrong?
You cannot translate Polish prepositions word-by-word, they work just for given contexts. When taking about what is eaten for any meal, in Polish we always use "na".
And BTW I would say, that the English preposition for is "extremely overused" compared do Polish, because it translates to a bunch of other prepositions in Polish, depending on context, and even within given context, their usage varies depending on verb/noun/pronoun involved. See:
In "Polish Grammar in a Nutshell" by Oscar E. Swan, there is a chapter about Polish Prepositions with a special paragraph about translations of "for". Here is also a good article about Polish Prepositions: "Everything You Need to Know about Polish Prepositions" by Adam Łukasiak.
A bit unnatural and would require a specific context, like eg. enumerating what a given person eats for each meal.