Learn Finnish: Vuori on liian korkea. - The mountain is too high.

The word liian means "too" (as in "excessively"). Just like in English, we say "too high" using the basic adjective, not "too higher". If you wanted to say "The mountain is higher", you would drop liian and add the -mpi suffix to the adjective: Vuori on korkeampi.

How exactly is the comparative form of "korkea" built?

To make the comparative, you take the inflectional stem of the adjective and add -mpi. For korkea, the stem is simply korkea-, so you just attach the suffix to get korkeampi (higher).

Can I use "korkea" to talk about a tall person?

No, korkea is only used for inanimate things like mountains, buildings, or concepts like prices. If you want to say that a person is tall, you need to use the word pitkä (which literally means "long").

Why is "korkea" in its basic form instead of the partitive case?

In Finnish, when you have a sentence structured as "A is B", the adjective's case depends on the subject. Because a mountain (vuori) is a single, countable object, the adjective describing it stays in the basic nominative form (korkea). If you were describing an uncountable substance like water or coffee, the adjective would take the partitive case.