Used after a preposed interrogative to introduce the remainder of the question.
Quand est-ce que les élèves retournent à l’école ?
When do the students return to school?
Combien de musées est-ce qu’il y a en France ?
How many museums are there in France?
Usage notes
Before a vowel, est-ce que becomes est-ce qu’. Example:
est-ce qu’elle
est-ce qu’il
In older forms of French, and in more formal registers of present-day French, the role of est-ce que is often fulfilled by subject-verb inversion:
Quand viendrez-vous ?
When will you come?
In fact, est-ce que itself results from subject-verb inversion; it comes from c’est que.
In colloquial French, yes-or-no questions are often indicated solely by punctuation (in writing) or intonation (in speech), with no special lexical or syntactic marker:
Tu es prête ?
You ready?
Similarly, non–yes-or-no questions often use the same structure as statements, with question words not being preposed:
Il a dit quoi ?
He said what?
In informal or colloquial French, question syntax is often used in indirect questions: