Learn Finnish: Sumutan kasvia, koska se pitää kosteasta ympäristöstä. - I mist the plant because it likes a moist environment.

The verb sumuttaa (to mist or spray) takes a partitive object because the action is ongoing, continuous, and doesn't represent a completed change of state. You are just applying some mist to the plant, not 'finishing' it.

Why do both kostea and ympäristö take the elative case (-sta/-stä)?

The verb pitää (to like) always requires the thing being liked to be in the elative case. Because Finnish adjectives must agree with the noun they modify, both the adjective kostea (moist) and the noun ympäristö (environment) get the ending. Due to vowel harmony, the endings look slightly different: kosteasta and ympäristöstä.

I often confuse pitää meaning 'to like' with pitää meaning 'to have to'. What is the grammatical clue here?

The clue is that the following words are in the elative case (-sta/-stä). When pitää means 'to have to', it requires the person doing the action to be in the genitive case (-n) and is followed by a basic verb, for example: Minun pitää sumuttaa kasvia (I have to mist the plant).

If I wanted to practice this lesson's grammar and add 'twice a week' to this sentence, how would I do it?

You would use the quantity followed by the time period in the inessive case (-ssa/-ssä). 'Twice a week' is kaksi kertaa viikossa. You would usually place it right after the object: Sumutan kasvia kaksi kertaa viikossa, koska se pitää kosteasta ympäristöstä.