Learn French: Le snob se pavanait au gala mondain comme s'il était riche. - The snob strutted at the high-society gala as if he were rich.

Just like in English ("as if he were rich"), French uses a past tense to express a hypothetical situation in the present. After comme si, you must use the imparfait (here, était) to describe an unreal state. You cannot use the conditional directly after si in this structure.

Why does comme si become comme s'il here? Does si always contract before a vowel?

No, the word si only drops its i and takes an apostrophe before il and ils. Before any other vowel (like elle or on), it remains intact. For example, "as if she were rich" would be comme si elle était riche.

I know monde means "world" or "people". What does mondain mean in gala mondain?

While related to monde, the adjective mondain translates to "high-society," "socialite," or "fashionable" in this context. A gala mondain is an exclusive, fancy social event for the upper class.

Why is the main verb se pavanait also in the imparfait instead of the passé composé?

While était is in the imparfait because of the comme si rule, se pavanait is in the imparfait because it describes an ongoing, continuous background action in the past. It sets a continuous scene of him walking around the party, rather than a single, completed action.