Learn Dutch: We gaan naar binnen, want het is donker. - We are going inside, because it is dark.

Want translates to "because", and it is a coordinating conjunction. This simply means it glues two normal sentences together without changing their structure. The word order in the second part stays exactly the same as a normal, independent sentence: Subject (het) + Verb (is).

Why do we say naar binnen instead of just binnen?

Binnen means "inside". However, when you are talking about movement toward a destination, Dutch adds naar (to). You literally say "We are going to inside" (We gaan naar binnen). If you were already resting inside, you wouldn't use naar, you would just say We zijn binnen (We are inside).

Do I always need a comma before want?

Yes. In Dutch, you almost always place a comma before want when it is used to connect two complete sentences together.

Why is "are going" translated simply as gaan? Shouldn't there be a translation for "are"?

English uses "are going" to show an action happening right now (the continuous tense). Dutch typically just uses the simple present tense for this. So, We gaan means both "We go" and "We are going". You do not need a separate verb for "are" here.