Every Spanish learner hits this wall. Ser and estar both translate to "to be", and choosing wrong changes your meaning entirely.

The short rule

Ser describes what something is — identity, origin, profession, permanent characteristics. Estar describes what something is like right now — location, mood, temporary states.

Why it matters

Es aburrido means "he is boring". Está aburrido means "he is bored". Same adjective, entirely different social consequences.

The trick that works

Ask yourself: would this still be true in a year? If yes, reach for ser. If it could change by tomorrow, use estar.