Learn Finnish: Haluatko kokeilla akupunktiota sen sijaan että ottaisit lääkettä? - Do you want to try acupuncture instead of taking medicine?

The phrase sen sijaan että (instead of) introduces an alternative action that isn't actually happening. Because taking the medicine is hypothetical or unrealized in this scenario, the verb following että requires the conditional marker -isi-.

Could I just use an infinitive verb like sen sijaan ottaa instead of creating a whole new clause?

In English you can simply say "instead of taking", but in Finnish, sen sijaan että must be followed by a full clause with a conjugated verb. You cannot just attach an infinitive directly to sen sijaan.

Why is the word for acupuncture translated as akupunktiota with a partitive ending?

The verb kokeilla (to try out or experiment with) is a partitive verb. It always requires its object to be in the partitive case because trying something is viewed grammatically as an incomplete or ongoing action.

Why is lääkettä also in the partitive case rather than the accusative?

Here, medicine (lääke) is treated as an uncountable mass noun. Since you are talking about taking some unspecified amount of medicine rather than one specific whole entity, the object must be in the partitive case.