Backshift

Backshift, literally shifting back, is a technical term in grammar for the change of a tense while changing the sentence from direct speech to reported speech. In a backshift, present tense is changed to past tense, and a past tense is changed to pluperfect (past perfect).

For example - He said,"I am sad." (direct speech).
He said that he was sad. (the tense is shifted back to past in reported speech.)
He said,"I ate fruit". (direct speech)
He said that he had eaten fruit. (the tense is shifted back to past in reported speech.)

Please note that backshift is not required, if the sentence in the direct speech is a general truth, or what was said is still true.