The term “sequence of tenses” refers to the choice of the verb tense in the subordinate clause depending on the tense of the verb in the main clause. The rule of the sequence of tenses means that the tense in the subordinate clause is determined by the tense in the main clause and should agree with it both logically and grammatically. The term "sequence of tenses" is often translated into Russian as “agreement of tenses”.
Generally, in complex sentences with all types of subordinate clauses, except the object clause, the sequence of the tenses in the pair “verb in the main clause – verb in the subordinate clause” is logical and based on sense and general rules of the use of tenses. The verb in the subordinate clause may be in any tense that reflects the actual time of the action and conveys the meaning correctly in the pair with the verb in the main clause.
e.g. She goes for a walk in the park when the weather is good.
She went for a walk in the park when the weather was good.
She has been teaching since she graduated from college.
I went to bed early because I was very tired.
He didn't answer your questions because he doesn't speak English.
While I was watching TV, the telephone rang.
By the time he returned, I had typed ten pages of my report.
The books that I bought yesterday are on my desk.
The surgeon who is going to perform the operation arrived yesterday.
The surgeon who was going to perform the operation fell ill yesterday.
Yesterday the patient felt better than he feels today.
Note: According to the rules of the use of tenses, the simple present is used instead of the simple future in adverbial clauses of time and condition referring to the future.
He will ask her about it when he sees her tomorrow.
She will visit them tomorrow if she has the time.
Sequence of tenses in sentences with object clauses
Object subordinate clauses answer the question “what?” and stand in the place of an object after such verbs as “know, think, believe, understand, wonder, agree, say, tell, ask, answer, remark” and phrases like “I'm sure (that), I'm afraid (that)”. Object clauses are connected to the main clause by the conjunction “that” (which is often omitted) and by other conjunctions, such as where, when, why, how, whether, if.
Present or future in the main clause
If the verb in the main clause is in the present or in the future, the verb in the object subordinate clause may be in any tense that conveys the meaning correctly according to sense, logic, and general rules of the use of tenses.
e.g. I think (that) he lives on Rose Street.
I don’t know if she is in town.
I don’t know whether he will agree to do it.
I wonder whether she will buy this house.
I’m not sure that he will help us.
I see that she is writing a report.
I know that John has already left for Chicago.
I know where she went.
I have heard that Mr. Smith is going to be our new director.
He will understand that you want to help him.
I will ask him why he didn't buy that book.
Past tense in the main clause
If the verb in the main clause is in the past tense, the verb in the object subordinate clause should also be used in one of the past tenses. The examples below show how the sentences given above will change if we use the past tense in the main clause.
e.g. I thought (that) he lived on Rose Street.
I didn't know if she was in town.
I didn't know whether he would agree to do it.
I wondered whether she would buy that house.
I wasn't sure that he would help us.
I saw that she was writing a report.
I knew that John had already left for Chicago.
I knew where she had gone.
I heard that Mr. Smith was going to be our new director.
He understood later that you wanted to help him.
I asked him why he hadn't bought that book.
This rule of the sequence of tenses may seem illogical to us, because the tense in object subordinate clauses doesn't always show the actual time of the action. In Russian object clauses, we are free to use any tense that conveys the meaning correctly, that is, the present, the future, or the past. But we have to use one of the past tenses in English object clauses if the verb in the main clause is in the past tense.
Exception from the rule
If a general truth is expressed in the object subordinate clause, the present tense is normally used in the subordinate clause despite the fact that the past tense is used in the main clause.
e.g. Newton discovered that the force of gravity pulls all bodies to the Earth.
Galileo proved that the Earth revolves around the Sun.
Compare the above examples with the standard cases below:
He learned that the hotel concierge usually locked the front door at midnight.
She found out that he still worked at a bank.
The choice of a past tense in the object subordinate clause
If the verb in the main clause is in the past tense (usually, in the simple past), there are three possible variants of the action in the subordinate clause: at the same time as the action in the main clause; earlier than the action in the main clause; later than the action in the main clause.
If the action in the subordinate clause took place at the same time as the action in the main clause, the simple past (or the past continuous if required by the context) is used in the subordinate clause.
e.g. I thought that he worked at a bank.
I knew that she was waiting for me by the entrance.
If the action in the subordinate clause took place earlier than the action in the main clause, the past perfect (or the past perfect continuous if required by the context) is used in the subordinate clause.
e.g. I knew that he had already left for Rome.
She said that she had been waiting for me for a long time.
If the action in the subordinate clause took place later than the action in the main clause, the future in the past is used in the subordinate clause ("would" is used instead of “will”).
e.g. I wasn’t sure that he would be at home.
I knew that she would be waiting for me by the entrance.
Reported speech (1) (He said that ...)
A. Study this example situation:
You want to tell somebody else what Tom said.
There are two ways of doing this: You can repeat Tom’s words (direct speech):
Tom said ‘I'm feeling ill.’
Or you can use reported speech: Tom said that he was feeling ill.
direct: Tom said:’ I am feeling ill.’ in writing we use these to show direct speech.
reported: Tom said that he was feeling ill.
B. When we use reported speech, the main verb of the sentence is usually past (Tom said that ... I told her that... etc.). The rest of the sentence is usually past too:
e.g. Tom said that he was feeling ill.
I told her that I didn't have any money.
You can leave out that:
e.g. Tom said (that) he was feeling ill.
I told her (that) I didn't have any money.
In general, the present form in direct speech changes to the past form in reported speech:
am/is -> was
do/does -> did
will -> would
are -> were
have/has -> had
can -> could
want/like/know/go etc. -> want/liked/knew/went etc.
Compare direct speech and reported speech:
You met Judy. Here are some of the things she said to you in direct speech:
‘my parents are very well.’
‘I'm going to learn to drive.’
‘John has given up his job.’
‘I can't come to the party on Friday.’
‘I want to go away for a holiday but I don’t know where to go.’ ‘I'm going away for a few days. I’ll phone you when I get back.’
Later you tell somebody what Judy said. You use reported speech
Judy said that her parents were very well.
She said that she was going to learn to drive.
She said that John had given up his job.
She said that she couldn't come to the party on Friday.
She said that she wanted to go away for a holiday but (she) didn't know where to go.
She said that she was going away for a few days and would phone me when she got back.
C. The past simple (did/saw/knew etc.) can usually stay the same in reported speech, or you can change it to the past Perfect (had done/had seen/had known etc.):
direct: Tom said: 'I woke up feeling ill, so I didn't go to work.'
reported: Tom said (that) he woke up feeling ill, so he didn't go to work. or Tom said (that) he had woken up feeling ill, so he hadn't gone to work.