Learn Finnish: Minä en tiedä.

In Finnish, the negative word "not" is actually a verb! It changes to match the person speaking. Since the subject is minä (I), the negative verb takes the form en. If the subject were "you", it would be sinä et.

Where does the form tiedä come from?

It comes from the dictionary form tietää (to know), which is a Type 1 verb. To make a negative sentence, you use the negative stem. You find this by taking the positive "I" form of the verb (tiedän) and dropping the final -n. This leaves you with tiedä.

Why is there a "d" in tiedä instead of the "t" from tietää?

This happens because of consonant gradation. In Type 1 verbs, the negative stem uses the "weak" grade. The strong -t- in the dictionary form tietää softens to a weak -d- when creating the personal stem, which carries over to the negative form tiedä.

Could I just say en tiedä and drop the word minä?

Yes! Because the negative verb en already makes it perfectly clear that "I" am the one who doesn't know, the pronoun minä is optional. In everyday speech, you will hear just en tiedä very often.