Read the following sentence, changing the verb from the present tense to the preterite tense. put the preterite form of the verb in the answer blank. yo sé la verdad.

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:

(I knew the truth)