This is from https://www.fun-mooc.fr/ A2 level chapter 4.8. The question is to select two alternate ways of asking "Combien de timbres vous voulez acheter ?"
The options are
Combien voulez-vous acheter de timbres ?
Combien est-ce que vous voulez acheter de timbres ?
Est-ce que vous voulez acheter combien de timbres ?
Est-ce que voulez-vous acheter combien de timbres ?
Only the first two are listed as correct. Why are the latter two incorrect?
When an interrogative word like combien is used alongside est-ce que, they always combine together and the interrogative word doesn't stay where it would be in a declarative sentence:
But never:
or:
You can think of it like est-ce que dragging the question word to the beginning of the sentence. The third and fourth answers violated this rule, which is the reason why they weren't correct.
Furthermore, est-ce que can never be used with inversion, making the fourth answer doubly wrong:
Extended comment over word order in interrogatives and the est-ce que particule
There's a handful of question words that don't exist in the formal language and can't be put at the beginning of a question. In those cases, they can't be used with est-ce que at all:
With combien de X, things are a bit more complicated because the question word can be either combien alone, in which case the de X part stays in situ, or the whole combien de X phrase:
As you can notice, est-ce que always goes after the whole interrogative word or phrase. It can't split it, so the sentence below is incorrect:
There are three ways to form a question in French: putting the subject after the verb (“Es-tu là ? Où vas-tu ? D'où viens-tu ?”), “est-ce que” (“Est-ce que tu es là ? Où est-ce que tu vas ? D'où est-ce que tu viens”), and the informal way of using the affirmative syntax with a rising tone at the end of the sentence (“Tu es là ? Tu vas où ? Tu viens d'où ?”).
With the first two ways, if there is a question word like où, quand, combien, comment, pourquoi, quel(le)(s), etc. then this question word must come at the beginning of the sentence. The only thing you can have before the question word is a preposition. Thus:
Combien voulez-vous acheter de timbres ? — Je veux acheter trois timbres.
Combien est-ce que vous voulez acheter de timbres ? — Je veux acheter trois timbres.
De combien de timbres avez-vous besoin ? — J'ai besoin de trois timbres.
De combien de timbres est-ce que vous avez besoin ? — J'ai besoin de trois timbres.
If the question doesn't start with a question word, it's a yes/no question. You can't turn it into an open-ended question mid-way.
Voulez-vous acheter des timbres ? — Oui.
Est-ce que vous voulez acheter des timbres ? — Oui.
There is one exception: in informal spoken French, you can form a question by using the normal affirmative syntax, and indicate that it's a question by using a rising tone. In this case, we normally put the question word at the same place that the answer would go.
Vous voulez acheter combien de timbres ? — Je veux acheter trois timbres.
Vous avez besoin de combien de timbres ? — J'ai besoin de trois timbres.
Even in this case, it's also possible to put the question word at the beginning.
Combien de timbres vous voulez acheter ? — Je veux acheter trois timbres.
De combien de timbres vous avez besoin ? — J'ai besoin de trois timbres.
