Learn French: Il nous méprise comme s'il faisait partie de la noblesse. - He despises us as if he belonged to the nobility.

After comme si (as if), we use the imparfait to describe a hypothetical or unreal situation in the present. He doesn't actually belong to the nobility, so this contrary-to-fact comparison requires the imperfect tense, just like in English (as if he belonged).

Why does si contract to s' in comme s'il?

The word si has a special rule: it only contracts and drops its vowel when followed directly by il or ils. It never contracts before other vowels, meaning you would write comme si elle or comme si on, but always comme s'il.

Could I use appartenait à instead of faisait partie de to say he belonged to the nobility?

You could, but faire partie de (to be a part of) is the most natural and common way to express belonging to a group, social class, or team in French. Appartenir à is more often used for strict ownership, like a physical object belonging to someone.

Why do we use de la before noblesse?