Learn French: Pourquoi est-ce que vous rejetez mon idée ?

The verb rejeter (to reject) belongs to a group of verbs that double their consonant in certain forms, just like jeter (to throw). However, you only double the t when the ending is silent (like je rejette). Because the -ez ending for vous is pronounced, the t stays single: vous rejetez.

Does the single T in rejetez change how it is pronounced compared to forms with a double T?

Yes! In forms with a double T like je rejette, the E before the T makes an 'eh' sound. In vous rejetez with a single T, that same E makes a soft 'uh' sound (or is dropped completely in fast speech).

I thought idée was a feminine word. Why does it use mon instead of ma?

You are right that idée is feminine! But because it starts with a vowel, saying ma idée would clash two vowel sounds together. To make it flow smoothly in speech, French swaps in the masculine mon before any feminine word that begins with a vowel or a silent H.

Could I ask this without using est-ce que?