Learn Finnish: Kysy kuljettajalta, tietääkö hän reitin. - Ask the driver if he knows the route.

The verb kysyä (to ask) requires the person you are asking to be in the ablative case, which ends in -lta or -ltä. In Finnish, you don't ask someone; you ask information from someone.

Why does the verb tietää have the -kö suffix here?

The -ko/-kö suffix is normally used to ask a direct yes/no question. However, in the middle of a sentence like this, it acts as the word "if" or "whether" to form an indirect question. So, tietääkö hän translates to "if he knows".

Why is the word for route (reitti) in the form reitin?

It is the object of the verb tietää. Since the subclause tietääkö hän reitin is a regular statement-like structure with a subject (hän), the total object takes the -n ending. The imperative verb kysy is in a completely separate clause, so its special object rules do not apply to reitti.

How is the imperative command kysy formed?

To form the singular imperative (giving a command to one person), you take the "I" form of the present tense and drop the -n. The dictionary form is kysyä, the "I" form is kysyn, so the command is simply kysy.