"I am eating a cheese sandwich."
Translation:Je mange un sandwich au fromage.
Actually, I can hear him say "omelette de fromage" and not "du" or his pronunciation is wrong.
In any event, both are impossible.
An omelette flavored with cheese is "une omelette au fromage".
"Une omelette de fromage" would suggest that you can make an omelette with cheese instead of eggs.
See https://www.thoughtco.com/a-vs-de-french-prepositions-4080520 , paragraph on defining and indispensable ingredient, and also here: https://french.kwiziq.com/revision/grammar/compound-nouns-are-formed-using-prepositions-a-de-en .
I'm slightly confused. If I heard someone say "I'm eating a sandwich with cheese," I'd wonder what kind of sandwich, because those are two different things, but the translation of 'sandwich au fromage' is 'cheese sandwich'. Wouldn't 'cheese sandwich' be structured similarly to 'robe niore' i.e. 'sandwich fromage'?
I agree that "de" being "of" would be the correct way to say this sentence. If i were to litteraly translate the sentence with "au", it would imply that the sandwich is "with" cheese, not that the sandwich is made "of" cheese as the word "de", would convey. But that is me making english logic from french, which may not work in this case.
Using "de" implies that the thing is made ENTIRELY of cheese. For example, le jus d'orange is made entirely of oranges. But café au lait is not made entirely of milk; it has some coffee in it, right? Using "à" tells you that there is a defining characteristic, but it is not the only ingredient.
Good question! "Cause it sounds too wordy for a simple statement. "...en train de..." is like, "in the process of" or "in the middle of," which are both very specific. Yes, if you could imagine a context in which someone said, I dunno, how about, "Honey! The phone's ringing. Could you answer it?" Then yes, in context, you could yell back, "I'm eating a cheese sandwich!" And then you could use "en train de." But here, we can assume (for lack of context) that it's just a regular statement.
Hope this helps!