The tense of a verb in the subordinate clause changes in accordance with the tense of the verb in the main clause.
The basic rules are as follows:
If the verb in the principal clause is in the present or the future tense, the verb in the subordinate clause may be in any tense, depending upon the sense to be expressed.
He says that he is fine.
He says that he was fine.
He says that he will be fine.
He will say that he is fine.
He will say that he was fine.
He will say that he will be fine.
If the tense in the principal clause is in the past tense, the tense in the subordinate clause will be in the corresponding past tense.
He said that he would come.
He told me that he had been ill.
I knew that he would not pass.
We noticed that the fan had stopped.
There are, nevertheless, a few exceptions to this rule.
A past tense in the main clause may be followed by a present tense in the subordinate clause when the subordinate clause expresses some universal truth.
Copernicus proved that the earth moves round the sun.
The teacher told us that honesty is the best policy.
He told me that the Hindus burn their dead.
A subordinate clause expressing place, reason or comparison may be in any tense, according to the sense to be expressed.
He didn’t get the job because his English isn’t good.
A fishing village once existed where now lies the city of Mumbai.
If the subordinate clause is an adjective clause, it may be in any tense as is required by the sense.
Yesterday I met a man who sells balloons.
Yesterday I met a man who sold me a balloon.
Note that when the subordinate clause is introduced by the conjunction of purpose that, the following rules are observed.
We use may in the subordinate clause when the main clause is in the present tense. We use might in the subordinate clause when the main clause is in the past tense.
I study that I may pass.
I will study that I may pass.
I studied that I might pass.
We eat that we may live.
He ate that he might not die.
If the principal clause is in the future tense, we do not use future tense in subordinating clauses beginning with when, until, before, after etc.
I will call you when dinner is ready. (NOT I will call you when dinner will be ready.)
I shall wait until you return. (NOT I shall wait until you will return.)
Expressions such as as if, if only, it is time and wish that are usually followed by past tenses.
I wish I was a bit taller.
It is time we started working.
He talks as if he knew everything.