I think of it in the context of Pasporta Servo. "Ĉu vi manĝos kun ni matene?" "Nu, kion vi manĝas por la matenmanĝo?"
Why is "la" in there but not translated and I get trouble if I forget the "the" elsewhere.
Because in English, we do not say, "What do you eat for the breakfast?" In many other languages, you do. Whereas, we do say, "What is the girl's name?" and in that case, "What is girl's name?" would be wrong.
I had it originally without the the but added it because in the past I
always got burnt by not including it!
Also, what if you are having a discussion about an event like The Esperanto Congress, which takes course over several days and make reference to the first day and want to know what was served at that first breakfast- is it not acceptable to say, "What do(did) you eat for the breakfast?"
I think the "la" makes sense if you look at the word "matenmanĝo" which, when deconstructed it reads "morning meal". So the sentence is just asking what you had for "the morning meal" in which case we already have the word breakfast. Other languages may not, so this covers all bases I guess.
Ĉu mi povus uzi, "What are you eating for your breakfast?" Duo ne akceptas ĝin. Mi volus registri la frazon, se ĝi pravas.
Is this a present tense question that you are asking someone currently eating food and you want to know what it is, ie "what are you currently eating for breakfast?" Or is this asking someone what they normally eat for breakfast?