Questions & Answers about Imam samo jedan ključ.
The I is already built into imam.
Imam is the 1st person singular present form of imati (to have), so it already means I have.
Serbian often leaves subject pronouns out when the verb makes the subject clear. You can still say Ja imam samo jedan ključ, but ja adds emphasis, as if you were saying I have only one key.
Because jedan has to agree with the noun it describes.
ključ is:
- masculine
- singular
So the correct form is jedan.
Compare:
- jedan ključ = one key
- jedna knjiga = one book
- jedno pismo = one letter