Learn French: Quelle chemise est-ce qu'il veut essayer ? - Which shirt does he want to try on?

Quelle is the feminine singular form of the question word for "which" or "what". It must match the noun it describes. Since chemise is a feminine noun, we use Quelle. If he were trying on a masculine item, you would use Quel instead.

Why is est-ce que placed in the middle of this sentence?

In French, a very common way to ask a question is to put the specific thing you are asking about (Quelle chemise) at the very beginning. You then follow it with est-ce que to signal that it is a question, and finally add the subject and verb (il veut).

Why is it written as qu'il instead of que il?

The word que at the end of est-ce que must drop its final vowel and use an apostrophe because the next word (il) starts with a vowel. This is a strict rule in French that helps the words flow together smoothly when spoken.

Does the verb essayer always mean "to try on"?

The verb essayer generally means "to try" or "to attempt" an action. However, whenever you are talking about an item of clothing, like a chemise, it specifically takes on the meaning of "to try on" to see if it fits.