Learn Farsi: آنها بیرون نمی‌روند تا اینکه باران بند بیاید. - They will not go outside until the rain stops.

The phrase تا اینکه (until) introduces a condition or limit that hasn't happened yet (the rain stopping). Whenever you use تا or تا اینکه to talk about an unrealized future event, the verb that follows must be in the present subjunctive.

The English translation says "They will not go", but نمی‌روند is in the present tense. Why isn't it in the future tense?

In Farsi, it is very common to use the present indicative (like نمی‌روند) for near-future actions or intentions. While you could technically use the formal future tense (نخواهند رفت), it would sound overly formal for everyday speech.

How exactly does بند بیاید mean "stops"?

It comes from the compound verb بند آمدن. The word بند refers to a tie, block, or dam, and آمدن means to come. Together, this specific compound verb is used for things that flow or fall but then cease, like rain, snow, or bleeding.

Could I just use تا instead of تا اینکه in this sentence?

Yes, you can just use تا. Both mean "until" in this context. Adding اینکه (literally "this that") simply puts a bit more emphasis on the full action that follows (the rain stopping) and sounds slightly more natural when introducing a complete clause.