Learn Finnish: Tuo puu on korkeampi kuin tämä talo. - That tree is taller than this house.

The base adjective is korkea (tall or high). To make it comparative (meaning "taller"), we add the suffix -mpi to the end. Since korkea ends in a vowel, the suffix attaches directly to the stem, giving us korkeampi.

What is the word kuin doing in this sentence?

In this comparative structure, kuin simply means "than". Just like in English, you use it right after the comparative adjective to introduce the second thing being compared, making korkeampi kuin translate to "taller than".

Why are both tuo puu (that tree) and tämä talo (this house) in their basic dictionary form?

When you compare two things using the word kuin, both items take the same grammatical case. Because the first item, tuo puu, is the subject of the sentence and in the basic nominative case, the second item, tämä talo, must also be in the basic nominative case.

Is there a different way to say "than this house" without using kuin?

Yes! Another very common way to form comparisons in Finnish is to drop the word kuin and put the second item into the partitive case. So, instead of kuin tämä talo, you could say tätä taloa, making the full sentence Tuo puu on tätä taloa korkeampi.