In spanish (castillian) gramar the Preterite or Simple Past Tense is used to express specific actions completed or concluded in the past that are separated from the present.
In other words:
It is applied when the person speaking gives information about the past that is not related to the present.
Now, in the case of the following sentence:
Yo sé la verdad
(I know the truth)
The preterite of the verb saber (to know) in the 1st person singular yo (I) is supe (knew).
Therefore, the sentence in preterite is:
Yo supe la verdad
(I knew the truth)
