Learn Italian: Torniamo al porto, a meno che la tempesta non passi. - We return to the port, unless the storm passes.

The conjunction a meno che (unless) almost always takes an extra, untranslated non right before the verb. This is called a "pleonastic" (filler) non. It doesn't actually make the verb negative; it's just a required grammatical pair. You can think of it as saying "unless the storm happens not to pass."

Why do we use passi instead of passa for "passes"?

The conjunction a meno che non expresses a hypothetical condition, so it always requires the subjunctive mood. Passi is the present subjunctive form of passare for the third-person singular (in this case, la tempesta).

Could I avoid the subjunctive and just say "if the storm doesn't pass"?

Yes, you can! You could say Torniamo al porto, se la tempesta non passa. Notice two differences: with se (if), you use the normal indicative mood (passa), and the non here is a true negative, literally meaning "does not pass".

The English says "We return", but could this also mean "We will return"?

Exactly. In Italian, the present tense (torniamo) is frequently used for plans in the near future. You could also use the future tense (Torneremo al porto...), but using the present is very natural and common in everyday conversation.