Learn Finnish: Ostan auton kahdeksantenatoista, mikä on iso ilo. - I am buying the car on the 18th, which is a big joy.

It can mean both! Finnish does not have a separate future tense. You use the present tense ostan (I buy / I am buying) for future events. The context, and time expressions like kahdeksantenatoista (on the 18th), make it clear that the action is happening in the future.

Why is auton used here instead of the partitive autoa?

Auton is in the genitive-accusative case because it is a total object. When you are doing an action to a whole, specific thing and the action will be completed (like buying one specific car), Finnish uses this case. If the action were ongoing or you were just buying "some car" without a clear completion, you would use the partitive autoa.

How does the word kahdeksantenatoista break down?

This means "on the 18th". The base word for "18th" is the ordinal number kahdeksastoista. To say "on" a specific date, Finnish uses the essive case, which adds the ending -na or -nä. For teen numbers (11 to 19), only the first part of the number gets the ending. So, kahdeksas changes to its essive form kahdeksantena, and the -toista is just added to the end.

Why use mikä for "which" instead of joka?

This is a very important rule in Finnish! You use joka when "which" or "who" refers to the specific noun right before it. You use mikä when "which" refers to the entire preceding sentence or idea. In this sentence, the "big joy" isn't the 18th itself, but the whole situation of buying the car.