Questions & Answers about Situacija je ozbiljna.
- situacija = situation
- je = is
- ozbiljna = serious
So the sentence literally matches English quite closely: Situation is serious, or more naturally, The situation is serious.
Je is the 3rd person singular present tense of the verb biti (to be).
So:
- ja sam = I am
- ti si = you are
- on/ona/ono je = he/she/it is
Since situacija is a singular noun, Croatian uses je = is.