Learn French: J'ai choisi un grand fauteuil pour ma chambre. - I chose a large armchair for my bedroom.

This sentence uses the past tense (passé composé). For regular -ir verbs like choisir (to choose), you form the past participle by dropping the -r. So, choisir becomes choisi. Combined with the helping verb ai, j'ai choisi translates to "I chose" or "I have chosen".

Why is grand placed before fauteuil?

Most French adjectives go after the noun they describe. However, a small group of common adjectives describing Beauty, Age, Goodness, and Size go before the noun. Because grand describes size, it is placed before fauteuil.

If a man is speaking, does he still say ma chambre?

Yes! In French, possessive words like ma (my) must agree with the gender of the noun that is owned, not the person speaking. Because chambre is a feminine noun, it will always be ma chambre, no matter who the bedroom belongs to.

Since the current lesson is about chez, could I use it in this sentence instead?

You could, but it changes the meaning slightly. If you replaced ma chambre with chez moi, the sentence becomes J'ai choisi un grand fauteuil pour chez moi (I chose a large armchair for my place/home). The original sentence simply gives more specific detail about which room the chair is for.