Learn Finnish: Ripustan kuvat kuivumaan vasta aamulla. - I hang the pictures to dry only in the morning.

Vasta specifically expresses a temporal delay, meaning 'not until' or 'only then'. If you used vain aamulla, it would mean you only hang them in the morning and never at any other time. Vasta aamulla means you are delaying the action and will not do it until the morning arrives.

Does vasta always go right before the time expression like aamulla?

Generally, yes. Vasta modifies the word or phrase that immediately follows it. Placing it right before aamulla clearly ties the delay to the morning. Moving it around can sound unnatural or shift the focus away from the delayed time.

Why is 'to dry' translated as kuivumaan rather than the basic infinitive kuivua?

The verb ripustaa (to hang) implies movement into a new state or location. In Finnish, when you leave or put something somewhere 'to do' something, you use the third infinitive in the illative case, which ends in -maan or -mään. You are hanging the pictures 'into' the state of drying.

Why is the object kuvat (the pictures) in the nominative plural instead of the partitive plural?