A sentence like "Elles écrivent de longues lettres" may confuse you. Why is there not "des longues lettres"? Had not we said, like, "Il vend des legumes"? What is the difference?
In fact, one usually says "de" when the corresponding noun is preceded with an adjective.
So it is: Elles écrivent de longues lettres.
Mais: Elles écrivent des lettres.
You made a little mistake when quoting "de longues lettres" again. You didn't leave out the S there. ;)
There is one important thing to know: if noun+adjective forms a new "word" you still use "des". You have to say "ce sont des grandes personnes" because "grande personne" doesn't mean "tall person" but "adult". There are some other expressions/words listed in the dictionary.
@Jola123: It is not a mistake, it is a question, why is there NOT "des" :)
Merci pour le complément !
Siebolt's post has it, but for the lazy, here's the relevant observation that seems to answer the question: "I was always taught in school that when the indefinite article "des" is followed by an adjective, it is replaced by "de". So "des petits chiens" is incorrect as far as I know. It should be "de petits chiens". "Des grands chats" is incorrect. It should be "de grands chats". In contrast, when the adjective comes after the noun, "des" is not replaced. "Des photos magnifiques" is the correct translation for "magnificent photos"." - AnnaMayaHouse