Learn French: Quand est-ce que tu t'endors ?

The verb here is s'endormir (to fall asleep). Like other daily routine verbs you are learning, it is reflexive. That means it needs an extra pronoun that reflects back to the subject tu (you), which is te. Because endors starts with a vowel, te drops its 'e' and combines with the verb to become t'.

What is the difference between dormir and s'endormir?

Dormir simply means "to sleep" (the continuous state of being asleep). By making it reflexive (s'endormir), the meaning changes slightly to "to fall asleep" or "to go to sleep" (the transition of starting to sleep).

Why do we use est-ce que here instead of just asking Quand tu t'endors ?

In everyday spoken French, putting est-ce que (literally "is it that") after a question word like Quand (when) is the most standard and natural way to form a question. While you might hear Quand tu t'endors ? in very casual street French, adding est-ce que is grammatically safer and sounds very natural to native ears.

Is s'endormir conjugated like a regular verb?