The future in the past

He was always going to be a botanist

You may like to look at the guides to future forms before tackling this guide. Then again, you may not. If you are content that you have a reasonable understanding of the seven or so forms English uses to talk about the future, then we can go on.

Reported or indirect speech vs. the future in the past

You should also bear in mind the guide to reported or indirect speech, linked in the list of related guides at the end, because a great deal of what is there impinges on this area. This is especially true for phenomena such as back-shifting tenses and the changes made to modal auxiliary verbs such as will. If that's unclear, see the guide.

There are times when we need to discuss plans that were made or undertakings given for the future seen from the viewpoint of the past.
For example, if you mentally form a current intention such as
    I'm going to call him tomorrow and then later want to discuss the intention, you might use an expression such as
    I was going to call him
That's not the same as using reported or indirect speech, but it is very similar because you are reporting your own thoughts.
By the same token, indirect speech often includes such examples as
    He said he would come
    He promised to write to me
    She undertook to have finished it by Friday
etc.
which are all ways of reporting what was said, written or implied.
This guide is not concerned with repeating what is explained in the one on indirect or reported speech.

The 5 main ways of talking about the future from a past perspective

There are five main ways English uses to see the future from the perspective of the past. All of them express the same views of the future that are expressed by similar constructions used either to talk about the future proper or to refer to some current intention, willingness or plan which will affect the future.
If you would like to remind yourself of the four most common ways to talk about future time in English, see the guide to four future forms, linked below.

was / were going to

This is sometimes called the unfulfilled intention construction. If you have an intention to do something but your plans are thwarted, you may well express the situation as something like:
    I was going to take a week off but there's too much to do
Equally, if someone uses will/shall to express current willingness such as:
    I'll let you have the money soon
but the promise is not kept, the person owed the money might well express the situation as:
    You were going to let me have the money soon
Notice how polite, even tentative, this can sound compared with, e.g.
    You promised to let me have the money

past progressive

This parallels the use of the present progressive used for plans and arrangement currently in place. For example, in narrative style we might find:
They were meeting in Paris the following week
which is not exactly reported speech but a very similar function conceptually.
This form is also often used to complain about arrangements broken or forgotten:
Did you forget that we were having lunch together today?

was / were about to

Used in the present, this marginal modal form implies an almost certain imminent action:
    I am about to lose my temper
    She is about to go to the airport
Occasionally, however, the action is cancelled or postponed (that's life) so we get a similar construction, often with but (to explain a reason) or when (to introduce an intervening act):
    I was about to lose my temper but he apologised so nicely that I calmed down
    She was about to go to the airport when the boss rang to say the trip was off
A parallel structure is another marginal modal, be on the point of, as in, e.g.
    I was on the point of losing my temper when she apologised
A similar but rather odd marginal modal auxiliary verb construction is went to which, when used in this way does not carry the normal sense of went at all. For example:
    I went to say something but she interrupted
The form is only used to refer to unfulfilled past intentions.

would

We are not here discussing forms such as
They said they would come at 6.
That is simply the change to the verb will used for predicted futures which is a common event when it is reported at a later time, back-shifting from will to would, its past form.

The modal would is used as the past of will in many situations where will is used to make firm future predictions. The difference, of course, is that when the prediction is in the past we usually know whether it turned out to be accurate. For example:

present prediction	past prediction
Computers will get faster and cheaper	Computers would get faster and cheaper
She will be a farmer	She would be a farmer
She won't be much good at it	She wouldn't be much good at it

The use of would to refer to a future in the past is also common in the context of projecting mental processes such as expect, think, imagine, guess, assume, hope, suspect etc. For example:

present projection	past projection
I suspect it will rain	I suspected it would rain
I hope she will see sense	I hoped she would see sense
They imagine I won't understand	They imagined I wouldn't understand

There's not much that's mysterious in either use because the form is closely analogous to the reported speech use exemplified above.

There is a rather formal use of would which refers to the future in the past. This form is commonly used in narratives, often to anticipate the plot in some way and grab the reader's attention.
Here's an example:
    She very soon would feel guilty about what she had done
The future forms proper that this parallels are not always easy to identify and quite varied but they include things like:
    She is going to feel guilty about what she has done
    She will feel guilty about what she has done
    She will be feeling guilty about what she has done

was / were + to

This parallels the formal obligation form. For example:
    The children are to come to school 10 minutes early next Monday
    You are to present the report at the next board meeting
If the obligation remains unfulfilled after the event, it is often discussed along with the reason for the failure using the conjunction but (or a similar concessive conjunct such as however etc.):
    The children were to come to school ten minutes early but the bus broke down
    I was to present the report at the next board meeting. However, it was cancelled at the last minute
See the footnote for another example.

Problems for learners

The fundamental issue for learners is that languages vary in how relationships between times are expressed. This is a conceptual issue as much as a grammatical one.
In English, we can refer to an event which is relative to another by using the aspectual system so, for example:
    I was going to telephone you later about this
in which the speaker is moving the point of reference into the past and expressing a future idea from there. The centre of this sentence is, therefore, the past, not the present. In technical terms, we can say that the speaker has shifted the deictic centre from its usual place (here, now and me) into the past.
Other languages take a different tack and, lacking relative or relational tense form, use adverbials instead or will use a simple past tense form to make this kind of statement such as:
    I wanted to telephone you later about this
which expresses only more or less the same idea.
Even comprehending what is meant by, for example:
    He was hosting a dinner party and left work early
is not straightforward for learners whose first languages do not have relational tense aspects, especially if they have been told that the past progressive form is used to signal on-going events in the past.

The upshot is that a good deal of error occurs when learners struggle to express the idea of a past intention, promise or arrangement. Examples are:
    *I will telephone you earlier today
    *I would telephone you earlier today
    *He is hosting a dinner party and left work early
    *He would host a dinner party and left work early
and more. All are caused by the inability to grasp the tense forms required in English to move the centre to the past.

time lines

None of the above (with the exception of the rarer would form) is particularly difficult to form but the concept of shifting the point of view (the deictic centre) into the past is somewhat mysterious to many learners.
Time lines help. In these, it is important that the eye is included because the forms are dependent on the way events and ideas are perceived. Here are examples of the time lines for all five ways of expressing a future embedded in the past:

was / were going to

past progressive

was / were about to

would

was / were to

Even with the use of time lines and other graphical representations, the need for constant concept checking remains.
For example:

She was going to present the report at that meeting
- When did she agree to present the report?
- When was the meeting?
- Did she present the report?
We were having lunch at The Ivy but we couldn't get a table
- What was arranged?
- Did they have lunch at The Ivy?
- Why not?
- What did they do?
Mary was about to leave the house when the telephone rang
- Where was Mary?
- Where exactly?
- What was she doing?
- What happened
- What did Mary do?
- Did Mary leave the house?
I said she would be angry and I was right
- What did I predict?
- Was I right?
- Is she angry now?
The President was to meet the Queen but she was ill
- What was arranged?
- Is the President with the Queen now?
- Why not?
- Where is the Queen now?

There is a very short test on this.

Related guides
reported or indirect speech	for the guide to the area which includes the reporting of future forms
four future forms	for a general guide to talking and writing about the future in English
Other tense forms
present forms	for the guide to talking and writing about the present
four more future forms	for consideration of a range of alternative future formulations
past forms	for consideration of a ways of talking and writing about the past
past perfect	for a guide to this area alone
present perfect	for a guide to a troublesome form
tenses and aspects	for the index to the whole area

Footnote:
For the forms under 4 and 5 above, you may like to consider this bit from Oscar Wilde:

She was thinking of Prince Charming, and, that she might think of him all the more, she did not talk of him, but prattled on about the ship in which Jim was going to sail, about the gold he was certain to find, about the wonderful heiress whose life he was to save from the wicked, red-shirted bushrangers. For he was not to remain a sailor, or a super-cargo, or whatever he was going to be. Oh, no! A sailor's existence was dreadful. Fancy being cooped up in a horrid ship, with the hoarse, hump-backed waves trying to get in, and a black wind blowing the masts down, and tearing the sails into long screaming ribands! He was to leave the vessel at Melbourne, bid a polite good-bye to the captain, and go off at once to the gold-fields. Before a week was over he was to come across a large nugget of pure gold, the largest nugget that had ever been discovered, and bring it down to the coast in a waggon guarded by six mounted policemen. The bushrangers were to attack them three times, and be defeated with immense slaughter. Or, no. He was not to go to the gold-fields at all. They were horrid places, where men got intoxicated, and shot each other in bar-rooms, and used bad language. He was to be a nice sheep-farmer, and one evening, as he was riding home, he was to see the beautiful heiress being carried off by a robber on a black horse, and give chase, and rescue her. Of course she would fall in love with him, and he with her, and they would get married, and come home, and live in an immense house in London. Yes, there were delightful things in store for him. But he must be very good, and not lose his temper, or spend his money foolishly. She was only a year older than he was, but she knew so much more of life. He must be sure, also, to write to her by every mail, and to say his prayers each night before he went to sleep. God was very good, and would watch over him. She would pray for him too, and in a few years he would come back quite rich and happy.
The lad listened sulkily to her, and made no answer. He was heart-sick at leaving home.
Yet it was not this alone that made him gloomy and morose.
The Portrait of Dorian Gray, Chapter 6

There are two aspects to note:

Wilde uses the were/was to form to talk in the past about Jim's imagined (rather than planned) future. This is an effective literary device.
In the final part, we know that she is actually saying all this to Jim because we have The lad listened so the would here is a form of reporting the verb will.