He, she, and it all take JE. They takes SU — but Serbian keeps three separate words for "they," one per gender. Here's the full picture, third person included.
You've already drilled sam/si/je/smo/ste/su as a block. This one zooms in on the part that trips people up: Serbian's three grammatical genders show up in the pronouns for "he/she/it" and "they," even though the verb form itself (je or su) doesn't change with gender.
on je — he is · ona je — she is · ono je — it is
oni su — they are (masc. or mixed) · one su — they are (fem.) · ona su — they are (neut., e.g. "deca")
1. Ja srećan.
2. Ti visok.
3. On doktor.
4. Ona učiteljica.
5. Ono malo.
6. Mi prijatelji.
7. Vi ljubazni.
8. Oni studenti.
9. One sestre.
10. Ona deca.
SR (lat): Odgovori nisu osetljivi na velika/mala slova, i može latinicom ili ćirilicom.
СР (ћир): Одговори нису осетљиви на велика/мала слова, и може латиницом или ћирилицом.
EN: Answers are not case-sensitive, and can be typed in Latin or Cyrillic script.
Igra, pa upotreba
Ista gramatika, drugo raspoloženje — Verb Run: BITI.