Learn French: Garde une voix ferme.

In the imperative (command) form, regular -er verbs like garder drop the final "s" for the tu (informal "you") conjugation. Even though you are talking to one person, tu gardes becomes just garde.

How would I change this sentence to say "Keep it" using the pronouns from this lesson?

Because voix is a feminine noun (une voix), you use the direct object pronoun la. In affirmative commands, this pronoun attaches to the end of the verb with a hyphen, making it Garde-la (Keep it).

If I wanted to say "Keep it firm", where does the adjective go?

You would replace the noun with the pronoun la, attach it to the verb, and leave the adjective right where it is. The sentence becomes Garde-la ferme (Keep it firm).

Why is the adjective ferme placed after voix instead of before it?

In French, most adjectives come right after the noun they describe. Only a specific group of short, common adjectives (usually describing Beauty, Age, Number, Goodness, or Size) go before the noun. Ferme doesn't fit those categories, so it goes after.

Does ferme change spelling if it describes a masculine noun?