What is the "correct" way of formulating questions? I just had the following task: Translate "Do they write a letter?" into French. What I inputted was:
Ils écrivent une lettre?
which is correct. However, another possible solution is Écrivent-elles une lettre? Of course, "Ecrivent-ils une lettre?" would be correct too. But what's the general difference between the "Ils ecrivent .." and the "Ecrivent-ils .." question style?
In general, a French person would say aloud "Est-ce qu'ils ecrivent une lettre?" using a phrase that essentially means "do they," but he or she would write "Ecrivent-ils une lettre?" The more commonly spoken phrase could also be "Qu'est-ce que" as in what is, for example "Qu'est-ce que c'est?" or "what is that?" Inversions (Ecrivent-ils versus ils ecrivent) are more common in written language. And the only difference between the ils and elles ecrivent answers is that "they" can be translated into they (all women) elles or they (either all men or a mixed group) ils.
Another common expression to ask question is "est-ce que".
In this example: "Est-ce qu'ils/elles écrivent une lettre ?"
(Beware, this is different from: "Qu'est-ce qu'ils/elle écrivent ?")