Learn Finnish: Onhan toisen laatikon muoto erilainen kuin toisen? - Isn't the shape of one box different from the other?

The phrase toinen ... toinen means "one ... the other". Both words take the case required by their role in the sentence. The first toisen is genitive because it possesses the box (toisen laatikon muoto, "the shape of one box"). The second one is also genitive because we are comparing the shape to that of the second box (kuin toisen, "than of the other").

Why does the sentence start with Onhan instead of just Onko?

Using the standard question clitic -ko (Onko...?) would make it a neutral yes/no question. Attaching the clitic -han instead turns the sentence into a question that expects the listener to agree, translating well to "The shape is different, isn't it?" or "Surely the shape is different?"

I thought kuin meant "than" for words like bigger or smaller. Why is it used with erilainen (different)?

In English you usually say "different from", but Finnish treats erilainen similarly to comparative adjectives like bigger or faster. Because of this, you always use kuin to make the comparison: erilainen kuin (different from).

Why isn't the word for box (laatikko) repeated at the end of the sentence?