Learn Finnish: Hän napsauttaa sormiaan, sillä hän haluaa saada sinut heräämään. - He snaps his fingers, for he wants to get you to wake up.

This is the core rule for expressing causation with saada. When you use saada to mean "to make/get someone to do something", the verb expressing the action they perform must be in the 3rd infinitive illative form, which ends in -maan or -mään. Think of it literally as getting someone "into the action of" waking up.

Why is "you" translated as sinut rather than the partitive sinua?

In this causative structure (saada + object + 3rd infinitive), the object is treated as a total object if the intended result is a completed action. Because he wants to successfully and fully wake you up, "you" takes the accusative case. For personal pronouns like sinä, the accusative form always ends in -t (sinut).

Why is sormiaan in the partitive case?

When you perform an action involving moving your own body parts (like snapping fingers, shaking your head, or waving your hand), Finnish usually requires the body part to be in the partitive case. Here, sormia is the partitive plural of sormi, and the -an ending is the third-person possessive suffix ("his/her").

Could I use koska instead of sillä in this sentence?