Learn Finnish: Traktorin rengas on leveämpi kuin auton rengas. - The tractor's tire is wider than the car's tire.

The word kuin works exactly like "than" in English. You place it right after the comparative adjective (leveämpi) and before the thing you are comparing against. When you use kuin, both things being compared are in the same case. Here, both traktorin rengas and auton rengas are in the basic nominative form.

How is the word leveämpi (wider) formed?

It is the comparative form of the adjective leveä (wide). To say "more [adjective]" or "[adjective]-er" in Finnish, you add the ending -mpi to the word's stem. Because leveä ends in a short vowel, we just add the suffix directly: leveä + -mpi = leveämpi.

Why do traktori and auto have an -n added to them?

The -n ending marks the genitive case, which shows ownership. It works exactly like adding "'s" to a word in English. So traktori becomes traktorin (the tractor's) and auto becomes auton (the car's).

Can I leave out the second rengas and just say "...kuin auton" like in English?