Learn Finnish: Leipä kovenee, kun se kylmenee. - The bread hardens when it gets cold.

These are Type 6 verbs. Their dictionary forms end in -eta or -etä. To find the stem, you drop the -ta or -tä and add -ne-. For the third-person singular (he/she/it) form, you double the last vowel of the stem. So kovene- + e becomes kovenee, and kylmene- + e becomes kylmenee.

What are the dictionary forms of these verbs?

They are koveta (to harden) and kylmetä (to get cold). Notice how Type 6 verbs are often built directly from adjectives: kova (hard) becomes koveta, and kylmä (cold) becomes kylmetä. They usually describe a change of state, meaning 'to become' that adjective.

How do I make these verbs negative to say 'The bread doesn't harden'?

To make the present tense negative, you take the negative word ei and add the verb's -ne- stem without any extra personal endings. So 'it doesn't harden' is simply ei kovene.

How would these verbs look in the past tense, as in 'The bread hardened'?