Learn Portuguese (Portugal): Desde que o meu amigo vendeu a casa, mora na cabana. - Ever since my friend sold the house, he lives in the cabin.

In this sentence, desde que means "ever since" (a point in time). When desde que is used to express time, it is followed by the indicative mood (like the simple past vendeu). If it meant "provided that" or "as long as" (a condition), it would require the subjunctive mood instead.

English uses "has been living" or "has lived" for this kind of sentence. Why is mora just in the simple present tense?

In Portuguese, when you talk about an action that started in the past and is still ongoing, you use the simple present tense. Because he moved into the cabin and still lives there now, mora (lives) is the correct tense to use.

Why is there no word for "he" before mora?

Portuguese frequently drops the subject pronoun (like ele) when the context makes it obvious who is doing the action. Since o meu amigo is the subject of the first clause, it is understood that he is also the one living in the cabin.

Just a quick refresher: why do we say o meu amigo and not just meu amigo?

In European Portuguese, it is standard to include the definite article (o or a) before a possessive pronoun like meu. You typically only drop the article when talking to the person directly, or with certain close family members.