Learn Polish: Co robisz po zajęciach? - What do you do after classes?

The preposition po, when used to mean 'after' an event, requires the Locative case. Because zajęcia is a plural noun, it takes the standard Locative plural ending -ach, becoming po zajęciach.

Is zajęcia a plural word? How do I say 'after class' in the singular?

When meaning 'classes' or 'school activities', zajęcia is only used in the plural. You cannot make it singular. To ask what someone is doing after their classes, you always use the plural form po zajęciach.

What is the difference between asking Co robisz po zajęciach? and Co zrobisz po zajęciach?

Using the imperfective verb robisz (you do) asks about a general routine, like 'What do you usually do after classes?' If you use the perfective future verb zrobisz (you will do), you are asking about one specific future event: 'What will you get done after classes today?'

Can I use po lekcjach instead of po zajęciach here?

Yes, but they have slightly different vibes. Po lekcjach specifically refers to finishing primary or high school lessons. Po zajęciach is broader and is typically used for university courses, seminars, or extracurricular activities like an art class.