寄生草莓的英文俱樂部

REF: http://usefulenglish.ru/grammar/sequence-of-tenses

Part 1. Sequence of tenses in complex sentences

Complex sentences with all types of subordinate clauses, except the object clause

1. ADV CLAUSES

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.

2. 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.

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Part 2. Sequence of tenses in sentences with object clauses

1. Present or future in the main clause

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.

2. Past tense in the main clause

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.

3. Exception from the rule

If a general truth is expressed in the object subordinate clause, the present tense is normally used.

Newton discovered that the force of gravity pulls all bodies to the Earth.
Galileo proved that the Earth revolves around the Sun.