Learn French: Le marquis a parlé de la maison comme s'il avait perdu sa propre fortune. - The marquis spoke about the house as if he had lost his own fortune.

In French, comme si (as if) is followed by the plus-que-parfait to describe an unreal or hypothetical situation in the past. Since the marquis speaking is already a past action (a parlé), the hypothetical loss of his fortune happens before or alongside it, requiring the plus-que-parfait (avait perdu). If the hypothetical situation were in the present, we would use the imparfait instead.

Why does si become s' before il in comme s'il?

The word si elides (drops the i and adds an apostrophe) only before the pronouns il and ils. It is a common trap for learners, but you must remember that it never elides before elle or on (so you must write comme si elle, never comme s'elle).

Does placing propre before the noun change its meaning?

Yes, the meaning of propre changes depending on its position. Placed before the noun as it is here, it means "own" (sa propre fortune means "his own fortune"). If it were placed after the noun, it would mean "clean" (for example, une chemise propre means "a clean shirt").

Why is it a parlé de la maison instead of using sur for "about"?