Learn Finnish: Hän työskenteli alusta loppuun saakka. - He worked from the beginning until the end.

While most postpositions require the genitive case, limit postpositions like saakka and asti are special. They require a local case. Because we are talking about a limit extending up to an endpoint (the end), loppu takes the illative case (loppuun).

Could I use asti instead of saakka in this sentence?

Yes, absolutely! When talking about time limits or endpoints, asti and saakka mean exactly the same thing. Saying alusta loppuun asti would be perfectly correct and natural.

alusta is in the elative (out of) case. Does saakka affect it too?

Yes, saakka (and asti) can be used with both the elative (from) and illative (to) cases. When you have a pair like alusta loppuun (from beginning to end), adding saakka at the end emphasizes the entire span, meaning all the way from the beginning right up to the very end.

What is the base form of työskenteli, and why does the stem change?