Page 111 (77, Should expressing probability), Oxford Learner's Grammar – Grammar Finder:
We can also use should to say that something is probable,
either in the present or the future.I posted the letter ages ago. They should have it by now.
The journey normally takes four hours, so we should get there about six.
In the negative we use shouldn't.
We're nearly at the front of the queue. We shouldn't have to wait
much longer.Should has the additional meaning of ‘if all goes well’.
There are no reports of delays. The train should be on time.
But we cannot use it to predict that something will go wrong.
There are reports of delays. The train will probably be late. [NOT
The train should be late.]
I understand this inferential usage of should and use it all the time. And I remember that would has such usage as well. It can be used to express presumption or expectation: That would be Steve at the door.
But few grammar books say much on this particular usage of would. It's probably the most intriguing and enigmatic usage of would, which prompts me to think when this usage is licensed in context: would it be possible to substitute would for should in the above examples?
I posted the letter ages ago. They would have it by now. #1
The journey normally takes four hours, so we would get there about six. #2
We're nearly at the front of the queue. We wouldn't have to wait much longer. #3
There are no reports of delays. The train would be on time. #4
There are reports of delays. The train would be late. #5
I don't think that "would" in all of these new examples have the same meaning as "should" in the original ones; so when is "would" licensed in the same way as in "that would be Steve at the door"?
Is this usage of "would" restricted to any verbs or contexts? I think it's not frequently used in this way.
Added:
9.8.3 Modal remoteness (from the Cambridge Grammar of the English Language, aka CGEL)
The tentative use:
i a. He'll be about sixty.
i b. He'd be about sixty.
The difference between present tense and preterite is much less tangible here than in (a) above: the preterite introduces a rather vague element of tentativeness, diffidence, extra politeness, or the like.
The intended context for [i] is that of answering such a question as How old is he? (we are not concerned here with the interpretation of [i b] as an implicit conditional, “He’d be about sixty now if he were still alive”).
In such a context the unmodalised He is sixty makes an unqualified assertion. Example [i a] is less assured: it involves what we have called the central-epistemic use of will.
The CGEL suggests it's a different usage of "would" from the hypothetical one, but fails to explain when "would" could be used that way.
Best Answer
These are all meaningful, but the meaning is changed and the context in which they could be used would be changed. "Would" is more often used to discuss expectations (that one feels certain about) in projected courses of action, rather than hopes or expectations in the current situation. You wouldn't use "would" there except in limited circumstances, when you are very confident in your predictions. I think a lot of this falls out of the use of "should" to describe model behavior or what is 'right' (like def. 3 here). Incidentally, I think (without proof) that that is why we don't use "should" to describe expectations that go awry, like the train being late: we certainly don't think that our hopes or expectations ought to be denied!
with "should" is a prediction; with "would" the speaker is certain enough to, for instance, discount the possibility of it not having been received. ("They would have it by now, so that can't be the reason they haven't responded...")
"should" states a probable expectation, while "would" is more like a certainty in a hypothetical plan ("We would get there by six, which would give us time to change before dinner...")
is tough. "We wouldn't have to wait much longer" is an acceptable utterance with an appropriate context: if I'm making a case for my plan (staying in line) over your plan (leaving the line). The implied full thought is "we wouldn't have to wait much longer [if we agreed to stay]." Here we are discussing the expected-to-be-certain details of possible plans, rather than stating our hopes about the future.
would work if you'd said "there were no reports of delays, the train would be on time [and that is why I am worried that your sister hasn't arrived yet]". This is related to sentence 1; I am stating a prediction about which I feel certain enough that I discount the train being late as a possible reason that your sister is delayed, and start worrying that something else could have happened to her.
Meaning is completely changed--we don't use "should" to emphasize things that happen contrary to expectation. Sometimes "The train would be late!" (with emphasis) is used expressing frustration that the train is late. The resulting statement is whining. A more full example: "The train would be late on the day I have an interview! This always happens to me!"
So: the distinction as I see it is pretty much what your edited addition says. "Should" suggests a more tentative attitude than "would," which may be from genuine uncertainty, or from an attempt to be more polite or more emotionally removed.
Both "should" and "would" are used to discuss an event about which the speaker is not completely certain or confident. But "would" seems to have the implication of talking about the details of a hypothetical world, future course of action, proposed plan, etc. "Should" is used to make less confident predictions about the future.