Learn French: Le marquis a ri comme s'il méprisait la noblesse. - The marquis laughed as if he despised the nobility.

After comme si (as if), we use the imparfait to describe an unreal or hypothetical situation happening at the same time as the main action. Even though the main action (a ri) is in the past, the hypothetical despising is simultaneous. If he had acted as if he had already despised them beforehand, we would use the plus-que-parfait (comme s'il avait méprisé).

Does the tense of the first verb (a ri) dictate the tense after comme si?

No. The tense after comme si depends entirely on its relationship to the main verb, not on the main verb's tense. To show a simultaneous hypothetical action, you always use the imparfait, whether the main verb is past, present, or future.

Why is there an apostrophe in comme s'il? Does si always drop the 'i' before a vowel?

The word si only drops its 'i' before the pronouns il and ils. Before any other vowel, including elle or on, it keeps the 'i'. For example, if it were a female subject, it would be comme si elle méprisait.

Is mépriser related to the word une méprise (a mistake)?

They look similar but have distinct meanings. Mépriser means to despise or to scorn, coming from mé- (bad/wrong) plus priser (to value). A méprise, however, is a misunderstanding or mistake, and comes from the verb se méprendre (to be mistaken).