Du and le
I always have breakfast-Duo says Je prends toujours le petit déjeuner. Do you always have coffee in the morning, Duo says Tu prends toujours du café le matin. Why wouldn’t it be Je prends toujours du petit déjeuner?
Coffee, as a liquid, is uncountable. Every morning, according to this sentence, you are not drinking a set amount of coffee, but rather an unspecified amount. In English, we can say either "Every morning I drink coffee" or "...I drink some coffee". In French, you must include the "some". This is the partitive "de".
Breakfast, on the other hand, is countable. It is treated as a discrete thing. Admittedly, in English we sometimes treat it as uncountable and say something like "I grabbed some breakfast on my way to school". However, in French it is not used that way. It is always countable, so it does not get the "de" in front of it in this sentence.
The answer to these questions is usually something along the lines of, "Because it's just that way."
I used to drive my teachers nuts asking, "Why ... ?" until my German teacher took me aside and told me what his French teacher told him. "There is no 'why' in language learning."
You might be able to scour a corpus of texts and find the moment people began writing x or y, but for a learner, it's more productive just to accept a language as it exists today and try to imitate it.
I say this with a lot of empathy. My German teacher did, too.