Learn Finnish: Henki jää auttamaan ketä tahansa.

The phrase kuka tahansa (anyone) acts like a single unit, but the first word still changes its case to fit the sentence. The verb auttaa (to help) always requires its object to be in the partitive case. The partitive form of kuka is ketä, which gives us ketä tahansa.

Why is the verb "to help" in the -maan form (auttamaan)?

The verb jäädä (to stay) requires the next verb to be in the third infinitive illative form, which ends in -maan or -mään. You can think of it as "stays into helping." You use this same form with movement verbs like mennä (to go) and tulla (to come).

Doesn't henki usually mean "breath" or "life"? How does it mean "genie"?

Henki is a very versatile word! While it often means "breath," "life," or "spirit," it is also the standard Finnish word for a magical spirit or genie, like the genie in Aladdin's lamp.

What happens to tahansa if I change the word order? Can I say tahansa ketä?

No, tahansa must always come directly after the question word it modifies. You can move the whole phrase ketä tahansa to a different spot in the sentence, but those two words must stay exactly in that order.