Learn French: Le syndic achète des stores. Il en achète trois. - The property manager is buying some blinds. He is buying three of them.

Le syndic refers to the property manager or the administrator of a co-owned building (similar to an HOA manager). Be careful not to confuse it with a labor union, which is a false friend: le syndicat.

Why do we need the pronoun en in the second sentence? Couldn't we just say Il achète trois?

In French, when you use a number to specify a quantity of a previously mentioned noun, you must use the pronoun en (meaning "of them"). You cannot leave the number dangling by itself as an object like you often can in English ("He is buying three"). It must be Il en achète trois.

How would you rewrite the first sentence to use the impersonal passive construction taught in this lesson?

You could say Il est acheté des stores par le syndic. In an impersonal passive construction, the dummy pronoun il (it) is used as the grammatical subject, followed by the passive verb (est acheté), and then the logical subject (des stores). Notice that the past participle acheté stays singular because it agrees with the grammatical subject il, not the plural blinds.

What is the difference between des stores and des rideaux?

Des stores specifically refers to window blinds (such as roller blinds or Venetian blinds), whereas des rideaux refers to fabric curtains or drapes.