Present Tense/Imperative Tense
 

I have looked at this over and over to make sure that I have accurately copied the correct answer for this translation question (Unit 98, Lesson 3 of 6).
The first verb is definitely salgas, but I cannot see why. There seems to be nothing imperative in the words, "when you leave" which I incorrectly translated, using the present tense - "Cuando sales de la galería, ve a casa inmediatamente. "

Question:When you leave the gallery, go home immediately.

Correct Spanish: Cuando salgas de la galería, ve a casa inmediatamente.

I see the necesity for the imperative form of ir, but not for the imperative form of salir. What am I missing?

In this context, when there is a situation that hasn't happened yet, you need the subjunctive. In this case, you haven't left the gallery, so this situation is yet to happen. We don't use the future tense, because there is an amount of uncertainty that you will ever leave the gallery. ;)

If you use the present tense with "cuando", you are talking about a habit. Following your exercise, so that it makes sense, you'd have a sentence like: "Cuando sales de la galería, vienes a casa inmediatamente". This means that every time you leave the gallery, you go back home immediately. It's something you always do.

Some other examples using "cuando" for a future event:
-(When your mother scolds you.) Cuando crezcas, me lo agradecerás.
When you're older you will thank me.
- Cuando tenga dinero suficiente, voy a comprar una casa.
When I have enough money I'll buy a house.
- Lávate los dientes cuando termines de comer.
Brush your teeth when you finish your meal.

Thank you for that, AngelicaDeAlquezar. It seems strange that the subjunctive would be thrown in at this level, with no mention of it in the guidebook. Just when I thought I couldn't face yet another verb tense, I find that the subjunctive is the same as the negative imperative, on which I have yet to gain a handle. (Emoji tearing out hair, here.) I'm hoping that such errors will be politely overlooked when I encounter some real, rather than digital, Spanish speakers.

Sometimes, language lessons introduce the use of some more advanced structures when there is another one being studied, so you will not really notice and learn it just as it is for similar sentences. That way, when you get to the actual lesson, you already saw how it's used and don't find it so hard to learn it.

Anyway, never be afraid of speaking. If you can focus on the right pronunciation of vowels, many other mistakes won't be a barrier for you to be understood. ;)