Learn Turkish: Et fırında çok yavaş pişer. - The meat cooks very slowly in the oven.

The verb root is piş- (to cook). To form the aorist (simple present) tense for general facts or habits, single-syllable roots ending in a consonant usually take the -ar or -er suffix. Because the vowel in piş- is an 'i', front vowel harmony makes the suffix -er.

I learned that some single-syllable verbs take -ir or -ur in the aorist. Why doesn't piş-?

It is true that there are about 13 special single-syllable verbs (like gel- or al-) that break the rule and take the -Ir style suffix. However, piş- is a regular single-syllable verb, so it follows the standard rule and uses -er.

Does yavaş mean 'slow' or 'slowly' here?

It means 'slowly'. In Turkish, you can often use an adjective like yavaş (slow) directly as an adverb to describe how an action is done. You do not always need to add a special suffix like we do with '-ly' in English.

Can you remind me why we add -da to fırın?

fırın means 'oven'. The -da suffix is the locative case, which tells us where something happens (in, at, or on). Because the last vowel in fırın is a back vowel (ı), vowel harmony requires us to use -da instead of -de to say 'in the oven'.