grammar / meaning “I was going to do the washing, but we would run out of washing powder.”

Question

The explanations of my grammar book is as follows. 1) “They left the house at 6.00 a.m. and would reach Edinburgh some 12 hours later.” (=they reached Edinburgh) 2) He was sure that the medical tests would show that he was healthy. (=we don’t know whether he was healthy or not) I think I understand sentence 2 (from the context of “he was sure”) but how do we know the fact (=they reached Edinburgh) in sentence 1?
And how about saying “and reached Edinburgh some 12 hours later.”? (because we know that the tense is “the future seen from the past” from the time expression of “12 hours later”) And is it possible for us to know whether something did happen or not in following sentences? Then how or why can we know? 1) “I was going to do the washing, but we would run out of washing powder.” 2) The exam was so easy that most people would have finished after 30 minutes. 3) So in my excitement I arrived early. I would putt some; maybe do some chipping; Likewise, how about saying (1)“we ran out of ---“, (2)”finished--- . I think that “would” in sentence (3) refers to “prediction” and we don’t know about the happening yet until the speaker tell us about the result. Am I wrong? Then may I know why?

Answer 1

"But we ran out of washing powder" is telling what happened, looking from afterwards (i.e., now).

"But we would run out of washing powder" is implicitly placing the temporal focus at the time I was going to do the washing,

As often with English tenses, the choice of tense is not forced by the sequence of events, but is to some degree under the command of the speaker, to talk about events in different ways, and focus attention on different times in the sequence.