Learn Swedish: Vem målade huset igår? - Who painted the house yesterday?

Målade comes from the verb att måla (to paint). Because it is a regular Group 1 verb, we form the simple past tense by adding -de to the infinitive. Verbs in this group always end in -ade in the past tense!

Why isn't there a helper verb like "did" in vem målade?

English often uses "did" in past tense questions, but Swedish just puts the main verb in the past tense. You simply say vem målade (who painted) without adding any extra helper words.

Why is the word for "house" written as huset here?

Because the sentence is about "the house", we need the definite form. Hus is an ett-word (ett hus), so we add -et to the end to mean "the", creating huset.

Is it okay to write igår as two separate words?

Yes! You will very often see it written as i går. Both igår and i går are perfectly correct ways to write "yesterday" in Swedish.