Learn Croatian: Kvadrat što sam ga vidio velik je, a trokut je mali. - The square that I saw is big, and the triangle is small.

Using što as an invariable relative pronoun (meaning "that" or "which") is an alternative to koji. When you use što to refer to a noun, you must include an extra pronoun inside the clause to show its grammatical case. Here, that is the accusative clitic ga (it) pointing back to the masculine noun kvadrat. It literally translates to "The square that I saw it".

Why is the enclitic verb je placed after velik instead of right after the subject?

The subject of the first half of the sentence is the entire phrase Kvadrat što sam ga vidio. While enclitics like je often follow the very first word of a sentence, they can also follow the first complete grammatical block. Because there is a natural pause after the long subject phrase, the adjective velik acts as the first word of the next intonation unit, so je tucks in right behind it.

Why does the sentence use a for "and" instead of i?

In Croatian, the conjunction a is used to show a contrast between two things, similar to "while" or "whereas" in English. Because the sentence is directly contrasting the size of the square with the size of the triangle, a is the most natural choice here instead of the simple addition word i.

Why is the adjective velik in its short form, but is in its long form?