Learn Portuguese (Portugal): Isto é a chave do irmão?

Portuguese doesn't have an equivalent to the English 's to show possession. Instead, you have to use the preposition de (of). Literally, this sentence is structured as "Is this the key of the brother?".

What exactly is do made of here?

do is a contraction (two words squished together) of the preposition de (of) and the masculine definite article o (the). Because irmão (brother) is a masculine word, it takes the article o. When you combine de + o, you get do.

Why use Isto for "this"? Shouldn't it be Esta to match the feminine word chave (key)?

Isto is neutral and never changes its ending to match a masculine or feminine word. It is used when pointing to a general, unidentified object or situation before naming it. If you were pointing directly at the key and attaching the word "this" to it, you would use the feminine word: Esta chave é do irmão? (Is this key the brother's?).

How do we know this is a question? The words are in the same order as a normal sentence.

In Portuguese, you don't need to swap words around or add helper words to the front of a sentence to make it a question. You simply use a questioning tone of voice that goes up at the end. In writing, the question mark does all the work!