Learn Croatian: Oni su mali i sretni. - They are small and happy.

In Croatian, "they" changes depending on who you are talking about. Oni is used for a group of males, or a mixed group of males and females. If it were a group of only females, you would use one instead.

What kind of word is su?

su is the short, unstressed form (a clitic) of the verb "to be" for the "they" form. You will use it most of the time for "are"! Because it is a clitic, it naturally wants to sit in the second position in the sentence, right after the subject Oni.

Why do the adjectives mali (small) and sretni (happy) end in -i?

Adjectives have to match the gender and number of the pronoun they describe. Because Oni is a masculine plural pronoun, the adjectives take the masculine plural ending -i. If we were talking about a group of females (one), they would be male and sretne.

Can I drop the pronoun Oni and just say su mali i sretni?

You can drop Oni, but you cannot say su mali i sretni. Short words like su (clitics) are never allowed to start a sentence. If you drop the pronoun, you have to rearrange the sentence so the clitic takes the second spot: Mali su i sretni.