Sometimes when we are discussing past events, we want to refer to something that was in the future at that time. In order to express this idea, we can use the past tenses of the verbs we would normally use to talk about the future. Thus,

is going to = was going to
I'm going to leave Britain to start a new life in Canada.
When I heard that she was going to leave Britain to start a new life in Canada, I was quite upset.

Other future verb forms change in the same way:

present progressive = past progressive:
We're meeting Jane outside the town hall at three o' clock.
We left school before the classes were over because we were meeting Jane outside the town hall at three o' clock.

will = would
If I play my CDs while I'm working, it won't disturb you, will it?
I didn't think my music would disturb her, but it did.

future progressive: will + be + verb-ing = would + be + verb-ing:
Don't phone them now, they'll be having supper.
I didn't want to phone them at that time because I thought they would be having supper.

future perfect: will have + past participle = would have + past participle:
I'll have finished writing up this report by noon, so we will be able to watch the tennis this afternoon.
I thought I would have finished that report by noon and then we could have watched the tennis, but it took longer than expected.

is to be = was to be; is about to = was about to:
The factory is to be closed down and all the work (is to be) transferred to Germany.
I was on holiday in Greece when I heard that the factory was to be closed down.
Please take your seats, ladies and gentlemen. The performance is about to begin.
We weren't able to finish our drinks. We had to take our seats as the performance was about to begin.