Learn Polish: Gdy pada deszcz, zawsze zostaję w domu. - When it rains, I always stay at home.

Zostaję comes from the imperfective verb zostawać, while zostanę comes from the perfective zostać. Because the sentence uses zawsze (always) to describe a repeated habit, you must use the imperfective form. Notice how the perfective base zostać changes into the imperfective zostawać by adding the -aw- suffix, which is exactly what you are learning in this lesson!

Why does the sentence say pada deszcz instead of just pada?

While you can absolutely just say pada to mean 'it is raining', adding the noun deszcz (rain) is very common in Polish. It literally translates to 'rain falls'. This is the standard way to talk about weather conditions, just like you would say pada śnieg for 'it is snowing'.

Could I use kiedy instead of gdy at the beginning of the sentence?

Yes, gdy and kiedy both mean 'when' and are completely interchangeable in this context. Gdy is just a slightly shorter, very common alternative used to connect two parts of a sentence like this.

Why is it w domu? Shouldn't a masculine noun like dom end in -e in the Locative case?