Learn French: Où est-ce que vous avez caché l'arme ?

In the passé composé, rien (nothing) is placed between the helper verb (ai) and the past participle (caché). You would say Je n'ai rien caché. If you wanted to say "I hid nobody," personne (nobody) goes after the past participle: Je n'ai caché personne. This is the key difference you are learning in this lesson!

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

Adding est-ce que after a question word like Où (where) is a very common way to ask a question in everyday French. It lets you keep the normal subject-verb order (vous avez) instead of flipping them around to avez-vous.

The English translation says "did you hide". Where is the word for "did"?

French does not use a helper word like "did" to make past tense questions. Instead, it uses the passé composé (avez caché). This tense translates to both "have hidden" and "did hide".

I see l'arme. Is the word arme masculine or feminine?

Arme is feminine (une arme). Because it starts with a vowel, the feminine article la drops its vowel and merges with the word to become l'arme, which makes it hard to tell the gender just by looking at it.