Du and De La/L'
When do you use "du" or "de la/l'"? Plus is it true that "de" and "le" can't be put together and you must use "du" instead?
de la is feminine, de l' is in front of words that start with vowels, and yes, du is always masculine and you cannot put de and le together without turning them into 'du'.
du and de la/l' are used as indefinite articles, and sometimes, if the word is uncountable, they play the role of 'some', e. g. "I have some bread" = "J'ai du pain". I eat some meat = Je mange de la viande.
For further rules, you can read this article: https://www.thoughtco.com/du-de-la-des-1368977
Under the Duolingo lesson headed "food", the grammar notes speak about this issue. I'll paste it below:
The Partitive Article
The partitive article is used for unspecified amounts of uncountable nouns. In English, it can translate to "some", but it's often just omitted. Remember that du is a contraction of de + le and that partitives can elide.
Gender Partitive Article Example Masculine du Je mange du poisson. — I am eating fish. Feminine de la Je mange de la viande. — I am eating meat. Elided Masc. de l' Je mange de l'ananas. — I am eating pineapple. Elided Fem. de l' Je bois de l'eau. — I am drinking water. Nouns almost never appear without articles in French, so articles must be repeated in serial lists.
Il cuisine du poisson et de la viande — He cooks fish and meat.