Learn Finnish: Uuni ei kuumene, jos ovi on auki. - The oven does not get hot if the door is open.

The basic verb is kuumeta (to get hot), which is a Type 6 verb. To find the stem for Type 6 verbs, you replace the -ta/-tä ending with -ne-, giving us the stem kuumene-. Since this is a negative sentence (ei kuumene), we use this bare stem exactly as it is, without adding any personal endings.

If the affirmative is uuni kuumenee (the oven gets hot), why is the negative ei kuumene with only one e at the end?

In the affirmative third-person singular (he/she/it), you double the final vowel of the stem (kuumene- + e = kuumenee). However, present negative verbs are always formed using the bare stem without that extra doubled vowel. So, the negative form drops back to just ei kuumene.

Is the verb kuumeta related to the adjective kuuma (hot)?

Yes, directly! A very common feature of Type 6 verbs is that they are created from adjectives to mean "to become [adjective]". Here, the adjective kuuma (hot) turns into the verb kuumeta (to become hot).

Why use auki for "open" instead of the adjective avoin?