Learn Finnish: Mihin laitoit magneetin ja vivun? - Where did you put the magnet and the lever?

In English, we say "Where did you put it?". But in Finnish, verbs of placing or putting, like laittaa, are treated as verbs of motion towards a destination. Because the items are moving to a location, you must use the illative case (mihin), not the static inessive case (missä).

Why do magneetti and vipu end in -n here?

They are the direct objects of the sentence—the things being put. Because this is a standard active sentence and the action is completed (a total object), the nouns take the accusative case. For singular nouns, this form looks exactly like the genitive case, which is why we see magneetin and vivun.

Why does vipu (lever) change to vivun?

When adding the -n ending, the final syllable becomes "closed" (it ends in a consonant). This triggers consonant gradation, causing the strong consonant p in vipu to soften to a v in vivun. You might remember this same p -> v change from words like lupa becoming luvan or tapa becoming tavan.