Learn Turkish: Yavaş ye.

In Turkish, giving a direct command to one person informally is done by using the bare root of the verb. The dictionary form of "to eat" is yemek. When you drop the infinitive ending -mek, you are left with the root ye.

The word yavaş means "slow", but the translation says "slowly". Is there no special ending for adverbs?

That is right! Unlike English, which often adds "-ly" to turn an adjective into an adverb, Turkish usually uses the exact same word. You just put yavaş right before the verb to describe how the action is done.

Could I say Ye yavaş instead?

You will be understood, but it sounds unnatural. Turkish strongly prefers putting the verb at the very end of the sentence. Words that describe the verb, like yavaş, naturally sit directly in front of it.