Sequence of Tenses
The sequence of tenses is the principle in accordance with which the tense of a verb in a subordinate clause follows the tense of the verb in the principle clause (sequence in latin is sequor (verb) which means 'follows').
The sequence of tenses applies chiefly to adverb clauses of purpose and noun clauses.
A past tense in the principle clause is followed by a past tense in the subordinate clause.
He hinted that he wanted money.
I found out that he was guilty.
There are however two exceptions to this rule.
1. A past tense in the principle clause may be followed by a present tense in the subordinate clause, when the subordinate clause expresses a universal truth.
He said that honesty is always the best policy.
2. When the subordinate clause is introduced by than, even if there is a past tense in the principle clause it may be followed by any tense required by the sense in the subordinate clause.
He liked you better than he liked me.
He helped him more than he helps his own children.
3. A present or future tense in the principle clause may be followed by any tense required by the sense.
He thinks that she is there.
He will think that she is there.
4. In sentences where the subordinate clause denotes purpose, if the verb in the principle clause is in present or in future tense, the verb in the subordinate clause may be in present tense.
I eat so that I may live.
I shall nurse him so that he may live.