Learn Croatian: To su slike i knjige. - Those are pictures and books.

If you look at their singular forms, slika (picture) and knjiga (book), they both end in -a. In Croatian, almost all nouns ending in -a are feminine. To make them plural when they are the subject of the sentence, you simply drop the -a and add -e.

Since slike and knjige are plural, why do we use the singular-looking To instead of a plural word for "Those"?

When you are introducing or pointing things out using the verb "to be" (like "This is..." or "Those are..."), Croatian uses the fixed, neutral word to. You never change it to match the gender or number of the nouns. Instead, you just change the verb: To je slika (That is a picture) versus To su slike (Those are pictures).

Why is the verb su placed exactly second in this sentence?

The word su (are) is an enclitic. Enclitics are short, "weak" words that do not have their own accent and can never start a sentence. In Croatian grammar, they strictly prefer to sit in the second position of a sentence, right after the first word or phrase.

Does the g in knjiga change its sound when it becomes plural?