What is difference between these two sentences:
- They left at 6 a.m. and would reach London after four hours.
- They left at 6 a.m. and reached London after four hours.
The book, Oxford Guide to English Grammar by John Eastwood, says 'would' can be used as the past form of 'will'. Is it always the case?
- They left at six am and would reach London after four hours.
This states that they did leave at 6am and did in fact arrive in London after four hours. However, the statement can be read so as to place the reader at a point in time after departure but before arrival. You could paraphrase the above to read (albeit less elegantly);
They left at six am. We know now that they arrived in London four hours later.
The statement about the time of departure is part of the narrative, but the arrival time is a point of fact highlighted to the reader, but out of context of the timeline in which the story is told.
- They left at six am and reached London after four hours.
This states that they did leave at 6am and also that they arrived 4 hours later.
The book, Oxford Guide to English Grammar by John Eastwood, says 'would' can be used as the past form of 'will'. Is it always the case?
Yes, I can't think of an exception to this, but there are of course other uses of the word would.
They left at six am and would reach London after four hours.
This means they left at six in the morning and were expected to reach London four hours later. Did they? We don't know, the sentence doesn't say it.
They left at six am and reached London after four hours.
Here it is clear that they left at six in the morning and they did arrive in London four hours later.
As for your second question, there are quite a few uses of the modal verb "would" and you can find explanations at The Use of the Modal Verb "Would" in a Certain Passage and a good review by Edwin Ashworth at The modal verb would
To me it has the following connotations:
They have already left (at six am) and will assumably arrive in London four hours after leaving (but might not have arrived yet)
They left at six am and arrived already, after travelling for four hours.
Disclaimer: not a native speaker.
The first sentence takes place when the subjects were on the train. While the reader knows that the train did eventually reach London, the sentence takes place at a point in time when the train had not yet arrived.
The second sentence describes the entire trip and takes place after the train arrived.
Consider if the subsequent sentence described the view from the moving train. It would seem natural following the first sentence, but jarring following the second.
