Questions & Answers about Miksi sinä ajat Porvooseen?
Why does Porvoo take the -seen ending here?
The illative (into/to) case ending depends on the vowels at the end of the word. When a word ends in two identical vowels, like the city name Porvoo, you add the ending -seen. That is why it becomes Porvooseen (to Porvoo).
Where is the word for "are" in "are you driving"?
Finnish does not have a separate continuous tense like English. The single verb ajat translates to both "you drive" and "you are driving". You do not need an extra word for "are".
Is it necessary to include the word sinä in this sentence?
No, it is optional! Because the verb ajat already ends in -t, which always means "you" (singular), you could simply ask Miksi ajat Porvooseen? and it would mean exactly the same thing.