Learn Dutch: Weet u waarop wij straks gaan schilderen? - Do you know what we are going to paint on later?

In Dutch, when you ask a question about a thing using "what" and a preposition (like op, meaning "on"), you cannot literally say "op wat". Instead, "what" turns into waar and attaches to the preposition. So, "on what" becomes the single word waarop.

Can I split waarop into two words in this sentence?

Yes, you can! It is very common in spoken Dutch to split these words. You could say: Weet u waar wij straks op gaan schilderen? When split, waar stays at the beginning of the clause, and the preposition (op) usually sits right before the verbs at the end.

Why are the verbs gaan schilderen at the very end?

This is an indirect question. Because the sentence starts with the main clause Weet u (Do you know), the rest of the sentence becomes a subordinate clause (a dependent phrase). In a subordinate clause, all the verbs are pushed to the very end.

The translation says "later" for straks. Could I just use the Dutch word later?

You could, but it changes the meaning slightly. Straks specifically means "in a little while" or "later today" (the near future). The Dutch word later is usually used for the distant future, like "years from now" or "when I grow up".