Learn Portuguese (Portugal): Ele quer ir pelas escadas.

Pelas is a contraction of the preposition por (meaning "by" or "through") and the feminine plural definite article as (the). Because escadas (stairs) is feminine and plural, we must combine them into pelas.

The English translation says "take the stairs". Why isn't there a verb for "take" in the Portuguese sentence?

In Portuguese, you don't "take" a route like stairs or a path. Instead, you use the verb ir (to go) followed by the preposition por (by/through). So, ir pelas escadas literally translates to "to go by the stairs".

Why is the verb ir not conjugated here?

When you have two verbs right next to each other that form one idea (like "wants to go"), only the first verb is conjugated to match the subject (Ele quer). The second verb stays in its infinitive or dictionary form (ir).

Why is escadas plural? Can I say pela escada?

When talking about a flight of stairs in a building, using the plural escadas is the most common and natural choice in Portugal. The singular word escada is more frequently used to mean a ladder.