Learn Turkish: Çadırı çabuk kur, yoksa ıslanırız. - Set up the tent quickly, otherwise we will get wet.

In this lesson's context, yoksa translates to "otherwise" or "or else". It connects the command (Çadırı çabuk kur) to the consequence that will happen if the command isn't followed (ıslanırız). It literally comes from yok (non-existent) plus the conditional suffix -sa (if), so it essentially means "if not".

Why is there an -ı at the end of çadırı?

That is the accusative case, which marks a specific direct object. Çadır means "tent". Because you are talking about setting up a specific tent ("the tent") rather than just any tent in general, you add the vowel -ı to make it çadırı.

Why is the verb kur just a short word with no endings?

Kur is the bare root of the verb kurmak (to set up, to establish). In Turkish, using just the root of a verb is how you give a direct command to one person informally. It is exactly like saying "Set (it) up!" in English.

The translation says "we will get wet", but ıslanırız doesn't use the future tense suffix -acak. Why?