Learn Finnish: Lentokoneen moottori on isompi kuin auton moottori. - The airplane's engine is bigger than the car's engine.

Kuin links the two things being compared. The phrase that follows kuin takes the same grammatical case as the thing it is compared to. Here, we are comparing the car's engine to lentokoneen moottori (which is the subject, in the nominative case). Therefore, auton moottori is also in the nominative case.

How is the comparative word isompi formed?

It is built from the adjective iso (big). To say "bigger" in Finnish, you add the comparative ending -mpi to the word's stem. Because iso ends in an o, the ending simply attaches to the base word without altering any letters.

Why do we use the genitive forms lentokoneen and auton here?

These words end in -n because they are in the genitive case, which shows possession or belonging, much like " 's " in English. So lentokoneen moottori literally translates to "the airplane's engine" and auton moottori translates to "the car's engine".

The word lentokoneen is a compound word. How do its parts change when we add the genitive ending?