Questions & Answers about Mä tiiän, että sun täytyy ehtiä junaan.
Why is the verb tiiän used here instead of tiedän?
This is a very common spoken Finnish (puhekieli) contraction. In spoken language, the letter d in the middle of a word is often dropped. So, the standard tiedän loses its d and becomes tiiän.
Why does the sentence use sun täytyy instead of sä täytyt?
The verb täytyy (to have to) is a necessive verb. These verbs never change endings to match the speaker. Instead, they always stay in the third-person singular (täytyy), and the person who has to do the action must be in the genitive case. Here, sun is the spoken genitive form of standard sinun.
Why is the word for train in the illative case (junaan)?
This is because of the verb ehtiä (to make it on time). In English you might simply catch a train, but in Finnish you make it on time into a destination. Because you are physically going into the train, ehtiä requires the destination to be in the illative case (junaan).
How would this exact sentence look in formal written Finnish (kirjakieli)?
You would need to swap out the colloquial pronouns and the contracted verb for their full standard forms. It would become: Minä tiedän, että sinun täytyy ehtiä junaan.