The verb in the subordinate clause changes its tense in accordance with the
tense of the verb in the main clause. This principle chiefly applies to adverb clauses of purpose and noun clauses.
Here are the basic rules
1. A past tense in the principal clause is usually followed by a past tense in the
subordinate clause.
- She said that she would come.
- I realized that I had made a mistake.
- I worked hard that I might succeed.
- I found out that he was guilty.
There are a few exceptions to this rule.
A past tense in the principal clause may be followed by a present tense in
the subordinate clause when the subordinate clause expresses a universal
truth.
- The teacher said that honesty is the best policy. (NOT The teacher said that honesty was the best policy.)
- Galileo maintained that the earth moves around the sun.
When the subordinate clause is introduced by than, it may be in any tense
even if the verb in the main clause is in the past tense.
- He loved me more than he loves his own children.
- He loved me more than he loved his own children.
- He loved me more than he will love his own children.
A present or future tense in the principal clause may be followed by any
tense in the subordinate clause.
- He says that she was at the club.
- He says that she is at the club.
- He says that she will be at the club.
- He will say that she was at the club.
- He will say that she is at the club.
- He will say that she will be at the club.
Notes
When the verb in the main clause is in the future tense, we often use a
present tense in the subordinate clause to refer to future time.
- I will call you when he comes. (NOT I will call you when he will come.)