Learn Croatian: Ova kuća je velika.

In Croatian, words for "this" have to match the gender of the noun they point to. Because kuća (house) is a feminine noun, which we can usually tell because it ends in the letter a, we must use the feminine form Ova.

Why does the adjective velika also end in an a?

Just like the word for "this", adjectives must match the gender of the noun they describe. The base dictionary word for "big" is velik, but because it describes the feminine noun kuća, we add an a to the end to make it feminine, giving us velika.

Why is je the third word? I learned it usually goes in the second position.

The word je (is) is a weak verb that prefers the second position in a sentence. Here, the phrase Ova kuća (this house) acts together as a single block or idea taking up the first position. Therefore, je takes the second logical spot right after it.