Learn Turkish: Lütfen yavaş konuş.

konuş is the bare root of the verb konuşmak (to speak). In Turkish, to give a direct command to one person (the informal singular "you"), you simply use the bare verb root without adding any suffixes.

Why do we use yavaş (slow) instead of a specific word for "slowly"?

In Turkish, adjectives can usually act directly as adverbs without adding a special ending like "-ly" in English. So yavaş means both "slow" and "slowly" depending on whether it describes a noun or an action.

Why does yavaş come right before konuş?

In Turkish, words that describe an action (adverbs) usually come immediately before the verb. Since the verb is typically at the end of the sentence, the describing word sits just in front of it.

Is using the bare root konuş polite as long as I say lütfen?

Adding lütfen makes it polite enough for friends or informal situations, but the bare root is still the informal "you" form. If you were speaking to a stranger, an elder, or in a formal setting, you would use the formal command form of the verb instead.