Creo que... I believe that...
Oigo que... I hear that...
Pienso que... I think that...
Es verdad que... It is true that...
Ejemplos:
Sé que el cielo es azul. I know that the sky is blue.
Creo que hay un examen. I think that there is a test.
Oigo que ella está enferma. I hear that she is sick.
Pienso que llueve. I think that it is raining.
Es verdad que nado bien. It is true that I swim well.