Learn Turkish: Gelenler kesin yanlış sokağa sapmıştır. - Those who are coming must have definitely turned down the wrong street.

The verb sapmış alone typically means "turned" (as a reported or indirect past event). By adding the epistemic suffix -tır to make sapmıştır, you express a strong deduction. Instead of just reporting a past event, you are saying "they must have turned" based on your own logical assumption.

If -mıştır shows an assumption, why use kesin (definitely) in the same sentence?

It is very common in Turkish to pair certainty words like kesin with the assumption suffix -tır. While -tır shows that you did not witness the event yourself and are deducing what happened, kesin emphasizes that you are completely confident in your deduction. Together they create the meaning "I am absolutely sure they must have..."

What is the grammar behind Gelenler meaning "Those who are coming"?

This uses the subject participle suffix -en. Gelen translates to "who comes" or "the coming one." By adding the plural suffix -ler, the participle acts as a noun meaning "the people who are coming" or "the arrivals."

Why does sokak (street) take the dative case to become sokağa?