Learn Finnish: Syön heti kun olen lisännyt ruokalusikallisen voita keittoon. - I will eat as soon as I have added a tablespoonful of butter to the soup.

In Finnish, when you have a sequence of two future events connected by heti kun (as soon as), the action that must be completed first is put in the perfect tense. It emphasizes that adding the butter will be completely finished before the eating begins.

Since 'I will eat' is in the future, why is the first word just 'Syön' (I eat)?

Finnish doesn't have a dedicated future tense. The present tense (syön) is used for future actions. The context, especially the sequencing with heti kun, makes it clear that this will happen in the future.

Why is 'ruokalusikallisen' (a tablespoonful) in the genitive case, but 'voita' (butter) is in the partitive?

Ruokalusikallinen is the total object of your action, so it takes the genitive-looking accusative case to show the action will be completed. However, when you specify what substance makes up a measurement, that substance (voi) must be in the partitive case (voita). Think of it as 'a tablespoonful of butter'.

Why do we use the illative case 'keittoon' (into the soup) instead of the inessive 'keitossa' (in the soup)?

The verb lisätä (to add) is a directional verb. Because you are moving the butter into a new place, Finnish requires the illative case (-Vn) to show movement into the soup.