Learn Finnish: Auto vietiin korjaamoon eilen. - The car was taken to the repair shop yesterday.

It is built from the verb korjata (to repair/fix). By taking the verb's stem and adding the location suffix -mo, we create korjaamo—literally "a place where repairing happens". In this sentence, it has the illative (into) case ending -on, making it korjaamoon.

Why is auto in its basic dictionary form (nominative) instead of an object case like auton?

Because the sentence uses the passive voice (vietiin). A major rule in Finnish is that when a complete object is acted upon in a passive sentence, it takes the basic nominative form rather than the standard accusative -n ending.

Could we use the -la suffix taught in this lesson and say korjaala instead?

No, because of the type of base word. The suffix -mo/-mö is usually added to verb stems to mean "a place where [verb] is done" (like leipoa -> leipomo). The -la/-lä suffix is typically added to noun stems to mean "a place associated with [noun]" (like kahvi -> kahvila). Since korjata is a verb, it takes -mo.

How is the verb form vietiin built from viedä (to take)?