Questions & Answers about Minun täytyy ajaa Joensuuhun tai Porvooseen.
Why does Joensuu become Joensuuhun, but Porvoo becomes Porvooseen? They both end in a double vowel!
This is a great example of two different illative rules! Porvoo is a multi-syllable word ending in a long vowel, which takes the -seen ending (Porvooseen). Joensuu, however, is a compound word (Joen + suu). The word suu (mouth) is a single syllable ending in a long vowel. Single-syllable words take the -hVn ending, so it becomes suuhun, making the whole word Joensuuhun.
Why does the sentence start with minun instead of minä?
The verb täytyy (must / have to) has a special rule: the person who has to do something must be in the genitive case (the -n ending). So instead of using the basic "I" (minä), you use "my" (minun). You can think of it as saying "It is necessary of me".
Since the sentence is about me driving, why isn't the verb ajan (I drive)?
Because of the verb täytyy. Whenever you use a "necessity" verb like täytyy, the action verb that follows it must stay in its basic dictionary form (the infinitive). So you use ajaa (to drive) instead of ajan.
If the ending means "to", could I just say "minun täytyy ajaa Joensuu"?