Quick answer
Long form before a noun: за́нятый челове́к. Short form as a predicate: Я за́нят. Short forms have four endings only: zero, -а, -о, -ы.
The pattern
| m. | f. | n. | pl. | Meaning |
|---|---|---|---|---|
| за́нят | занята́ | за́нято | за́няты | busy |
| свобо́ден | свобо́дна | свобо́дно | свобо́дны | free |
| гото́в | гото́ва | гото́во | гото́вы | ready |
| уста́л | уста́ла | уста́ло | уста́ли | tired |
| бо́лен | больна́ | больно́ | больны́ | ill |
The masculine often inserts a vowel to make itself pronounceable: свобо́дный → свобо́ден, бо́льный → бо́лен. And the feminine frequently pulls the stress to the ending: занята́, больна́.
Which adjectives have them
Not all do. The productive group is states and conditions: рад (glad), до́лжен (must), уве́рен (sure), согла́сен (in agreement), прав (right), похо́ж (similar), ну́жен (needed). Several of them have no long form in this meaning at all — рад and до́лжен are short-only.
Long or short?
With a noun, always long: свобо́дное ме́сто. As a predicate about a temporary state, short: Ме́сто свобо́дно. A long form as predicate is possible but shifts the meaning to a permanent property: Он больно́й suggests a chronic condition, Он бо́лен means he is ill right now.
Hear the stress
The feminine занята́ with end stress is the single most audible marker of a short adjective. Learners who say за́нята are understood, but it sounds instantly off.
Typical mistakes
Я за́нятый.
Я за́нят.
A current state takes the short form.
Она́ за́нята.
Она́ занята́.
Feminine short form, stress on the ending.
Check yourself
Which is correct?