The Sequence of Tenses is the principle in accordance with which the tense of the verb in the subordinate clause follows the tense of the verb in the principal.
The main rules are :
Rule 1: If the verb in the Principal clause be present or future, the verb in the Dependent clause may be in any tense ; as,
Rule 2. But if the verb in the Principal clause be in the past tense, the verb in the Dependent clause should be in the corresponding past tense ; as,
But when the Dependent clause expresses universal truth or a habitual fact, the verb is not changed :
Rule 3. When the Dependent clause denotes comparison, or does the work of an adjective, any tense in the Principal clause may be followed by any tense in the Dependent :