Explain the two main rules of sequence of tenses - SS2 English Structure, Speech Work, Structure,Vocabulary Question

There are two main rules of sequence 

1. The tense of the subordinate clause must be consistent with the tense of the main clause.
2. The tense of the subordinate clause must be consistent with the meaning of the sentence.

For example, if the main clause is in the past tense, the subordinate clause must also be in the past tense. However, if the main clause is in the present tense, the subordinate clause can be in the present tense, the past tense, orthe clause must be consistent with the meaning of the sentence.