Learn Finnish: Kahvi kylmeni nopeasti, kun olimme ulkona. - The coffee got cold quickly when we were outside.

It comes from the Verb Type 6 word kylmetä (to get cold). To find the stem of a Type 6 verb, you drop the -eta/-etä and add -ne-, giving us kylmene-. To form the past tense for 'it' (the coffee), we add the past marker -i. The -e- at the end of the stem drops out before the -i, resulting in kylmeni.

Is the verb kylmetä related to the adjective kylmä?

Yes! This is a very common feature of Verb Type 6. These verbs often mean 'to become [adjective]'. Here, the adjective kylmä (cold) turns into the verb kylmetä (to get or become cold). Other common examples are lyhetä (to become short, from lyhyt) and paheta (to become worse, from paha).

If the coffee didn't get cold, how would we say 'The coffee did not get cold'?

It would be Kahvi ei kylmennyt. For the negative past tense of a Type 6 verb, you use the negative verb (ei) and the past participle of the main verb. For Type 6, you form this by replacing the basic form's -eta/-etä with -ennyt/-ennut (or thinking of it as adding -nnyt to the -ne- stem), giving us kylmennyt.

Could I use ulos instead of to mean 'outside' in this sentence?