Learn Finnish: Vaimo ei lukitse ovea. - The wife does not lock the door.

Lukita (to lock) is a Type 5 verb. To build the stem for these verbs, you replace the ending -ita with -itse-. This gives the stem lukitse-. For negative sentences, you simply use this bare stem after the negative word (ei).

Why is the word for door in the partitive case (ovea) here?

In Finnish, if a sentence is negative (she does not lock), the object of the action must be in the partitive case. Ovea is the partitive form of the basic word ovi (door).

How would you say "The wife locks the door" in the affirmative?

It would be Vaimo lukitsee oven. To make the affirmative "she" form, you take the stem lukitse- and double the final vowel to get lukitsee. Also, because the sentence is now affirmative and the action is completed, the object changes from the partitive (ovea) to the accusative form (oven).

What is the "I" form of this verb?

It is lukitsen (I lock). You simply take the same stem used in the negative form (lukitse-) and add the personal ending -n.