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Collins COBUILD English Grammar

5.134 You use should with have to say that you expect something to have happened already.

Dear Mom, you should have heard by now that I’m O.K. ( I would add a little more context here: "Mom, I saw the doctor talking to you. You would have heard by now that I am OK." Otherwise, there are two explanations for this example, which could bring deviation from what I'm asking here. )

You also use should with have to say that something was expected to happen, although it has not in fact happened.

Muskie should have won by a huge margin.

By default, you use would with have to talk about actions and events that were imaginary or hypothetical in the past.

The hypothetical usage of "would" is pervasive, though the inferential usage is much less common.

I think should is almost interchangable with would in the second example.

I was wondering if I could use would in place of should in the first example without considerable change in meaning.

A single yes or no would suffice.

  • This is how I would understand it (which is probably not very different from your understanding): a) Dear Mom, you would have heard by now that I’m O.K. (= Mom didn't hear/know that), b) Dear Mom, you should have heard by now that I’m O.K. (= I expect, based on my knowledge and experience, that it's very likely that Mom's heard that. How likely is it? It's about somewhere between might have heard and must have heard.) – Damkerng T. Apr 13 '15 at 11:00
  • Yes, exactly. I just wanna know if would have done could bear a realis significance as well. @DamkerngT. – Kinzle B Apr 13 '15 at 12:10
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No, "should" is not generally interchangeable with "would". The easiest rule of thumb is that when a speaker uses "should" she is prepared to explain why, and when a speakers uses "would" she is prepared to explain why not. Here are some examples:

Muskie should have won by a huge margin [because he is much better than the competition].

Muskie should win tomorrow [because he is much better than the competition].

You should have heard by now that I'm OK [because I saw the doctor talking to you].

You really should have started that paper more than 8 hours before it was due [because it's very difficult to write a quality paper in less than 8 hours].

With "should", the speaker always has a reason why something did/will occur. If the speaker's reason [in brackets] isn't explicitly stated, it is still implied. Now consider "would":

Muskie would have won by a huge margin [if he didn't blow his engine].

You would have heard by now that I'm OK [but you didn't buy me a cell phone, so I couldn't call you].

I would have started that paper earlier [if I wasn't so busy with all my other homework].

With "would", the speaker always has a reason why not - why something did not occur. The speaker is more likely to explicitly state her reason [in brackets] with "would". If we are using a future tense, the why not rule of thumb becomes awkward but still workable:

I think she would like this as a gift [if we decide to buy it for her].

In sentences like this, everyone understands that it is hypothetical, so the speaker would rarely actually say the reason in brackets [if this hypothetical situation ever arose]. (See what I did there!)

One final note, which makes this kind of tricky, is that you can still use "should" even when you state a why not reason as long as a why reason is still stated or implied:

Muskie should have won if he didn't blow his engine.

Actually means:

Muskie should have won if he didn't blow his engine [because he is much better than the competition].

So in sentences like this, "would" and "should" are almost interchangeable. Just remember that "should" means the speaker knows why and "would" means the speaker knows why not. If the speaker happens to know both, then she can choose "should" or "would" almost equivalently.

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Using "would" changes the connotation, in that the reader expects to hear why the expected thing did not happen (or the conditions under which, in the speaker's view, the thing would have happened.)

For example,

Or

  • Mom, you would have heard by now that I was OK, but the pack of ravenous wolves that attacked us also ate my cell phone, which is why I didn't call and tell you why Rachelle and I aren't back yet from the movie, even though it ended three hours ago.

(You might argue that "should" invokes this same curiosity, and indeed the writer should address that eventually. But with "would", the reader expects an immediate explanation.)

  • You didn't even mention the first usage of "should" here. Your answer seems to be an endless loop to me. The implicit conditional you mentioned is not what I'm asking here. – Kinzle B Apr 13 '15 at 6:55
  • Your first example begs the question, using should and would interchangeably, but claiming they are different. And you said you "would" add a little context, but the context that you "did" add only made the example more confusing, not less. So I simply addressed the question of whether "should" and "would" are ever interchangeable, and offered counterexamples for the hypothetical usage. If you can fix your first example, perhaps someone will address the other usage. – Brian Hitchcock Apr 13 '15 at 7:54
  • I had to use another example ell.stackexchange.com/questions/21161/…, isn't "would have" similar to "should have" there? – Kinzle B Apr 13 '15 at 8:31
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You should have heard by now that I am OK.

The sentence isn't a hypothetical or imaginary conditional sentence in which you use "would have"in the main clause with the subordinate clause in the past perfect when you are talking about events in the past.

The sentence with "should have" indicates that You expect something to have happened alteady.

So the phrase "should have" isn't interchangeable with "would have".

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