Learn Dutch: Wij blijven binnen, want het onweer is hevig. - We are staying inside, because the thunderstorm is severe.

While you are currently learning about subordinating conjunctions like tenzij (unless) that push the verb to the end of the clause, want (because) belongs to a different group. It is a coordinating conjunction, which means it simply links two normal main clauses. Because of this, the word order in the second half stays as Subject-Verb: het onweer is. If you used the other word for because, omdat, the verb would move to the end: ...omdat het onweer hevig is.

The English translation says "are staying", so why don't we use a continuous form like zijn aan het blijven?

In Dutch, the continuous aan het form is rarely used with verbs that describe a state or a lack of motion, such as blijven (to stay). The simple present tense wij blijven is the most natural way to say "we are staying" here.

Why use binnen for "inside" instead of in?

binnen is used here as an adverb meaning "indoors" or "inside". in is a preposition, which means it needs an object to point to (for example, in het huis, meaning "in the house"). Since we aren't specifying a building and just mean "indoors", binnen is the correct word.

Is there a rule for why onweer gets the article het?