Learn Finnish: Sen sijaan että ottaisin särkylääkettä, kokeilen tätä yrttiä. - Instead of taking a painkiller, I will try this herb.

In Finnish, the phrase sen sijaan että (instead of) introduces an action that you are not actually going to do. Because this action is hypothetical and contrary to fact, Finnish requires the conditional mood marker -isi-. Literally, it translates closer to 'Instead of the fact that I would take...'

Can I just use a noun directly after sen sijaan että, for example 'sen sijaan että särkylääkettä'?

No, sen sijaan että must always be followed by a full clause containing a conjugated verb. If you just want to say 'instead of a painkiller' using only a noun, you drop että and put the noun in the genitive case before the postposition sijaan, giving you särkylääkkeen sijaan.

Why is särkylääkettä (painkiller) in the partitive case here?

In this context, medicine is treated as an uncountable substance. You are taking an unspecified amount of medicine ('some painkiller') rather than a complete, countable object, so it takes the partitive case.

Why is tätä yrttiä (this herb) in the partitive case?

This case is strictly determined by the verb kokeilla (to try out or experiment with). It is a partitive verb, meaning it always requires its object to be in the partitive case, regardless of whether it is a whole herb or just a piece.