should have been & must have been

[vicky1008]

Hi guys,

'Hope you could help me! I'm at my wit's end in teaching the difference between "should have been" and "must have been". This was once hotly
discussed in this forum: (http://www.englishpage.com/forums/archive/index.php/t-9933.html)
I read all the posts in the thread, but this is still not clear to me.
The problem is as follows:
Fill in the blank so that the two sentences are the same in meaning.
A. - I'm sure that he knew about it.
- He ( ) have known about it.
The correct answer is, according to the excersize book, "must".
American Heritage® Dictionary of the English Language says:
should - Used to express probability or expectation: They should arrive at noon. must - Used to indicate logical probability or presumptive certainty: If the lights were on, they must have been at home.

How about these?
B. - I think that he knew about it.
- He ( ) have known about it.
Would native speakers feel the answer SHOULD :Dbe "should", and not "must"?
C - I'm absolutely sure that he knew about it.
- He ( ) have known about it.
In this case, the answer MUST be "must", and not "should"?
Is it just the degree of assertion, and "must" is more assertive than "should"?
In the case of Question A, either "must" or "should" could be the answer?

Thanks,
vicky

[Marius Hancu]

A: both work, each with the meaning given in your quotations.

If you make an assumption of something which you assume to be certain/sure for/of the past, use "must have." Both can be used in your sentences, depending on context.

----
Must is used with the perfect infinitive (have + past participle) to express certainty about the past.

Edna isn't in her office. She must have gone home.
(= It seems certain that she has gone home

Swan, Practical English Usage, must
----

[Marius Hancu]

A. - I'm sure that he knew about it.

Be aware that there's an ambiguity at work here:
- He should have known about it. [It was his obligation to know about it
OR:
- He should have known about it. [I think with a high probability that he had to know about it, that it was logical for him to know about it.

A. - I'm sure that he knew about it.
- He must have known about it. [I'm pretty certain/sure (that) he knew about it.

[Marius Hancu]

C - I'm absolutely sure that he knew about it.
If the following isn't an answer, but a reformulation of the above:
- He definitely must have known about it.

[OddThomas]

One problem with must and should is an inherent problem of equivocation, or multiple meanings. In addition to their roles as modals to indicate likelihood versus certainty, both are often used to obligate. Must is also used to compel, command, or resolve, and it often expresses inevitability. These multiple meanings of must and should cause serious problems in determining whether we are resolving a simple matter of probability, or interpreting a more complex semantic issue.

That said, the use of must relies on certainty, while the use of should requires only high likelihood. The reason for choosing must or should is to convey information to the listener, not to illuminate the speakers knowledge. The listener has to come away from the conversation with enough information to act. Must tells the listener certainty; should tells the listener doubt. When the speaker forms the sentence, this is the goal. To reach this goal, determine first whether you are certain, then select must or should.

Examples: A. - I'm sure that he knew about it. (I'm dead certain.)
- He (must) have known about it.

B. - I think that he knew about it. (I'm uncertain.)
- He (should) have known about it.

C - I'm absolutely sure that he knew about it. (Again, I'm dead certain.)
- He (must) have known about it.
If you want to waffle, and begin to explain how you're sure, then you're not sure.I'm sure that he knew about it. I saw him pick up the note, and I feel certain he read it before the wind blew it away. Anyway, he saw Alice get in the car with John later, and he would have said something then, I know. And then Brad said that nasty thing about Glenda, so that tells you what he knew. So there's no way that Hamilton didn't know. He must have known about it.
Should is the better word above, no doubt. Of course, if you mean that after all that falderal he was obligated to know, or that it was inevitable that he knew, then choose must. But be aware that you have left the realm of should/must modals expressing a degree of certainty.

[Marius Hancu]

I strongly suggest to really compare the entries for should and must in the best dictionaries you have.

Cobuild Advanced Learner's would be a good place to start for ESLers.

Even these are OK

http://www.merriam-webster.com/dictionary/should
http://www.merriam-webster.com/dictionary/must

for your questions.

Another book I'd recommend is Swan, Practical English Usage. Deals with such stuff.

[vicky1008]

Thanks Marius, Odd Thomas for your tips. They are very helpful.

This is an exercize question for senior high school students in Japan.
Of the meanings of "should", my students understood the one to indicate
"obligation", but were not sure about the difference between "should"
suggesting "probability/expectation" and "must". This is partly because
the Japanese translation of "should" and "must" is confusing, I think.

Anyway, now this is clear to me. Thank you!