Learn French: Le professeur nous la montre sous le microscope. - The professor shows it to us under the microscope.

When using double pronouns in French, the first and second-person pronouns (me, te, nous, vous) always come before the third-person direct object pronouns (le, la, les). If both pronouns were third person, the order would flip to direct then indirect (for example, la lui).

Since this lesson is about the passive voice, how would we make this sentence passive, and would we use de or par?

We would use par and say: Elle nous est montrée par le professeur. The preposition de is reserved for the passive voice of verbs describing states, emotions, or conditions (like aimé de or couvert de). Because montrer is a specific, deliberate action, the agent must be introduced by par.

What does la refer to here, and how does it translate to "it"?

The pronoun la is a direct object replacing a feminine noun mentioned earlier in the conversation (perhaps la cellule or la bactérie). Even though objects like cells don't have gender in English and we just translate it as "it", French always requires the pronoun to match the gender of the noun it replaces.

Could we say au microscope instead of sous le microscope?