Sequence of tenses

If the verb in the principal clause is in the past tense, the verb in the subordinate clauses, too, will be in the past tense.

• She told me that she was ill. (NOT She told me that she is ill.)
• I knew that he would come. (NOT I knew that he will come.)
• He realized that he had forgotten to take the keys. (NOT He realized that he has forgotten to take the keys.)

There are some 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 some universal truth or general facts.

Study the sentences given below.

• Copernicus proved that the earth moves round the sun. OR Copernicus proved that the earth moved round the sun.
• The teacher said that honesty is the best policy. OR The teacher said that honesty was the best policy.

As you can see, both present and past tenses are possible in these cases. However, the present tense is more appropriate.

If the subordinate clause is an adverb clause

If the subordinate clause is an adverb clause of place, reason or comparison, even if the verb in the main clause is in the past tense, it can be followed by any tense.

• A great city once stood where now lies the village of Rajgir.
• He failed because he is weak in English.

In subordinate clauses introduced by when, until, before, if and after, we use a present tense to refer to the future.

• I will call you when dinner is ready. (NOT I will call you when dinner will be ready.)
• She will come if you invite her. (NOT She will come if you will invite her.)
• I will finish the report before I leave. (NOT I will finish the report before I will leave.)