serio

Es un asunto serio que hay que tratar con cuidado.

It is a serious matter that needs to be handled carefully.

Hablas en serio o me estás tomando el pelo?

Are you being serious or are you pulling my leg?

Su cara sería me indico que algo iba muy mal.

His serious face told me that something was very wrong.

Mi abuelo era un hombre muy serio y trabajador.

My grandfather was a very serious and hard-working man.

Tienen una relación seria desde hace tres años.

They have been in a serious relationship for three years.