Learn Croatian: Majka pere posuđe i rublje svaki dan. - Mother washes the dishes and laundry every day.

Because the phrase svaki dan (every day) tells us this is a repeated, habitual action. In Croatian, you must use imperfective verbs for actions that happen regularly or repeatedly. A perfective verb, like opere, would mean she successfully finished washing them on one specific occasion.

If she is washing multiple dishes and clothes, why are posuđe and rublje in the singular form?

Posuđe (dishes) and rublje (laundry) are collective nouns. They refer to a group of items but are treated as singular neuter nouns in grammar. They are the direct objects here, and because they are neuter, their accusative form looks exactly the same as the nominative. This is very similar to how we treat the word 'laundry' as singular in English!

What case is the time phrase svaki dan?

It is in the accusative case. When you want to express when or how often an action happens (like 'every day', 'this morning', or 'all year'), Croatian commonly uses the accusative case without any prepositions.

The verb is pere, but the dictionary form (infinitive) is prati. Why does it change so much?