Learn Croatian: Nisam znao kolika je ova brana! - I didn't know how large this dam is!

While koliko is an adverb meaning "how much" or "how many" (and doesn't change form), kolika is the feminine nominative form of the adjective koliki. As an adjective, it means "how large" or "how great," and it has to agree in gender, number, and case with the noun it describes. Since brana (dam) is a feminine noun in the nominative case, we must use kolika.

The English translation says "how large this dam is", but the Croatian sentence starts in the past tense (Nisam znao). Why don't we use the past tense for the dam too?

Unlike English, Croatian doesn't generally use strict "sequence of tenses" rules where the subordinate clause shifts to the past just because the main clause is in the past. Because the dam is still large right now, Croatian naturally uses the present tense (je) in the second part of the sentence, even after a past tense verb like znao.

Why is the verb je placed right after kolika instead of at the end of the sentence?

The word je is an enclitic (a short, unstressed word), which means it usually sits in the second position of its clause. The subordinate clause starts with the exclamatory adjective kolika, which takes the first position. Therefore, je must immediately follow it, jumping ahead of ova brana.

If a woman is speaking, does kolika change to match her gender?