Learn Finnish: Sinä et pese autoa huomenna. - You do not wash the car tomorrow.

This sentence is negative (et pese), and in Finnish, the object of a negative sentence must always be in the partitive case. To make auto partitive, we add an -a to the end, making it autoa.

Why is the verb written as pese and not peset if it means 'you wash'?

When you make a sentence negative, the negative word takes the personal ending. Here, et shows the 'you' form. The main verb that follows must drop its personal ending (the -t) and just use its base stem instead.

The dictionary form of 'to wash' is pestä. How do we get pese?

Pestä belongs to a verb group (Verb Type 3) where the ending -stä changes to -se- when forming the stem. That gives us the stem pese, which is exactly what we use after the negative word et.

How does Finnish show that this action happens in the future?

Finnish does not have a specific future tense like the English word 'will'. You simply use the present tense verb (et pese) together with a future time word like huomenna (tomorrow) to show when the action happens.