Learn Finnish: Minä en avaa ovea. - I am not opening the door.

Avata is a Type 4 verb. To find the negative form for Type 4 verbs, you take the dictionary form and simply drop the final -ta or -tä. So, avata minus -ta becomes avaa. Then you just add the negative word en in front of it to make en avaa.

Why is the word for door ovea instead of the basic form ovi?

Ovea is in the partitive form. In Finnish, when a sentence is negative (like not opening a door), the object of the action must almost always be in the partitive. Since you are not opening the door, ovi changes to ovea.

Could I drop the word Minä and just say En avaa ovea?

Yes, you can! The negative verb en already tells us that "I" am the one not doing the action. In spoken Finnish, people often keep the pronoun, but grammatically it is perfectly fine to drop Minä.

The translation says "I am not opening", but can this also mean "I don't open"?

Exactly! Finnish does not have a separate continuous tense like English does with "am opening". Depending on the context, en avaa can mean either "I am not opening" right now, or "I do not open" in general.