Seguro vs. cierto
While both seguro and cierto can be translated to mean sure or certain, they focus on different aspects of certainty. The word seguro is primarily about personal confidence, conviction, or the guarantee of an outcome. On the other hand, the word cierto emphasizes objective truth, facts, and reality.
Seguro
The word seguro is used to express personal certainty, conviction, or confidence that a belief is correct or an event will occur. It is highly associated with a person's state of mind, often used with the verb estar, or used to describe a guaranteed outcome.
Estoy seguro de que dejé las llaves en la mesa de la cocina.
(I am sure that I left the keys on the kitchen table.)
Ella no está muy segura de querer ir a la fiesta esta noche.
(She is not very certain she wants to go to the party tonight.)
Es seguro que lloverá mañana por la tarde según el pronóstico.
(It is certain that it will rain tomorrow afternoon according to the forecast.)
Estamos completamente seguros de nuestra decisión financiera.
(We are completely sure about our financial decision.)
¿Estás seguro de que este es el camino correcto hacia el museo?
(Are you sure this is the right way to the museum?)
Me parece casi seguro que ganaremos el partido de hoy.
(It seems almost certain to me that we will win today's match.)
El doctor está seguro de que el nuevo tratamiento funcionará rápidamente.
(The doctor is sure that the new treatment will work quickly.)
Cierto
The word cierto is used to state that something is an undeniable fact, objectively real, or completely true. Instead of focusing on someone's personal feeling of confidence, it points out that the information itself is accurate and verified.
Es cierto que la empresa va a contratar más empleados el próximo mes.
(It is certain that the company is going to hire more employees next month.)
Lo que dijo el testigo en el juicio de ayer es completamente cierto.
(What the witness said at yesterday's trial is completely true.)
No es cierto que el gerente haya cancelado la reunión de la mañana.
(It is not true that the manager canceled the morning meeting.)
Resultó ser cierto el rumor sobre la repentina renuncia del director.
(The rumor about the director's sudden resignation turned out to be true.)
Es muy cierto que el tiempo pasa volando cuando te diviertes.
(It is very certain that time flies by when you are having fun.)
¿Es cierto que te vas a mudar a otro país el año que viene?
(Is it true that you are going to move to another country next year?)
Todos pensaban que era una broma, pero el anuncio resultó ser cierto.
(Everyone thought it was a joke, but the announcement turned out to be certain.)
Summary
In short, you should use seguro when describing someone's personal confidence or a guaranteed outcome, and use cierto when confirming that a specific statement or situation is an objective truth or fact.