Learn Finnish: Minä kirjoitan laskun heti. - I will write the bill immediately.

This is the genitive-accusative object ending. In this sentence, writing the bill is a complete, finished action, meaning you are going to write the whole bill. To show this total completed action in a normal affirmative sentence, we add an -n to the object.

The translation says I will write, but isn't kirjoitan present tense?

Yes! Finnish doesn't have a separate future tense word like will. Instead, it uses the present tense along with context. Here, the total object (laskun) combined with the present tense verb shows that the action will be carried out to completion in the future.

What is the base dictionary form of laskun?

The base form is lasku, which means bill or invoice. Since it ends in a single vowel (-u), we just attach the -n directly to the end of it to make it the object.

Could I drop the word Minä and just say Kirjoitan laskun heti?

Absolutely! Because the verb kirjoitan already has the -n ending that means I, the pronoun minä is completely optional. Including it just adds a little bit of emphasis.