Learn Finnish: Isä siirtää painavan repun takakonttiin. - Father is moving the heavy backpack into the trunk.

This is the illative case, which means into. To make this form, you usually double the last vowel and add -n. Because takakontti (trunk) ends in -i, we double the i and add -n, making it takakonttiin (into the trunk). It answers the question mihin (where to/into).

Why do we say painavan repun instead of painava reppu?

The heavy backpack is the object of the sentence, meaning it is the thing being moved. In Finnish, when you do something to a whole object, it often gets the -n ending. Notice that the adjective painava (heavy) has to match the noun, so it also gets the -n ending.

What happened to the double p in reppu when it became repun?

This is because of consonant gradation. When we add the -n ending to reppu, we close the last syllable. This causes the strong double pp to weaken into a single p, making it repun.

What kind of word is takakontti?

It is a compound word made of taka (back or rear) and kontti (container). Together, they are used to mean the trunk of a car.