Learn French: Le marquis a regardé le snob comme s'il était un monstre. - The marquis looked at the snob as if he were a monster.

In French, the phrase comme si (as if) must be followed by the imparfait to describe a hypothetical situation happening at the same time as the main verb. Even though it expresses a hypothetical idea, French grammar strictly forbids using the conditional directly after si.

The main action is in the past (a regardé). Why doesn't that force the second verb into the plus-que-parfait?

The tense after comme si depends on its relationship to the main verb, not the main verb's tense itself. Because the marquis imagining the snob as a monster happened at the exact same time he looked at him, you use the imparfait. You would only use the plus-que-parfait if the imagined state happened before the looking (for example, if he looked at him 'as if he had been a monster earlier').

Why is it comme s'il instead of comme si il?

The word si always drops its i to form a contraction (s') when it is directly followed by il or ils. This is a mandatory rule. However, remember that si never contracts before elle or elles, so 'as if she were' would be comme si elle était.

Is snob a masculine noun because it refers to a man here, or is it always masculine?