Dear teachers,

1) If I say:

a) It's 7 p.m., he should be at home now.
b) It's 7 p.m., he must be at home now.

Both modals can be used here, right? But what's the nuance between them? What's exactly the difference between ASSUMPTION and DEDUCTION.

2) But in the following example, these modals cannot be interchangeable and "SHOULD" can't be used here, right? What should would mean here: OBLIGATION?

Phineas, as he heard this, remembered former days in which he had ridden about Saulsby Woods, and had thought them to be anything but hateful. “Is Saulsby shut up?” he asked.
“Altogether, and so is the house in Portman Square. There never was anything more sad or desolate. You would find Mr Kennedy altered, Mr Finn. He is quite an old man now. He was here in the spring, for a week or two — in England, that is; but he stayed at an hotel in London. He and Laura live at Dresden now, and a very sad time they must have / should have.”

3) now, "MUST HAVE" can be replaced by "MUST BE HAVING" and the meaning wouldn't change. But what about changing "SHOULD HAVE" into "SHOULD BE HAVING"? Would it make a difference?

Thank you very much for your help.
1. To me, a) shows less certainty than b)-- that's it.

2. 'What a very sad time they should have' makes no sense to me, because the speaker is very confident of his assertion. If the situation were more conditional, 'should have' would work: 'what a very sad time they should have if they were trapped in Yellowknife all winter'.

3. 'Must be having' works to make 'must have' more animated, and 'should be having' does the same for 'shold have' in my hypothetical context.
1. He [should / must] be home now.

What is the difference between assumption and deduction, you ask, meaning the difference between "should" and "must", I assume.
I would ask, "What is the difference between expectation and deduction?", but the question is still nearly the same.

Assumption or expectation, whichever we think of when we talk about the meaning of "should", indicates "I am confident he is home now" "One would think he was home now" "It would be reasonable to assume/expect that he is home now".

(There is an alternate reading of "should": that of obligation. In this reading, we are saying that he is obliged to be home now. It is his duty to be home now. This alternate reading does not seem to be part of your concern.)

Deduction, or "logical deduction" if you will, thought of as the meaning of "must" says, "There can be no other logical conclusion from the evidence but that he is home." "We know from the logical consequences of the facts that he is home now."

2. Yes. In this example, the "should" would be the "should" of obligation, and as such inappropriate for the context. There is no question that the author does not mean that it is the duty of Mr. Kennedy and Laura to "have a very sad time". The author means that there is only one possible conclusion, given the facts surrounding the case: Mr. Kennedy and Laura are having a very sad time.

3. "must be having" can substitute for "must have", yes. "should have" is not used here. It is inappropriate. Likewise "should be having" is equally inappropriate.

Hope that helps!

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Thank you, Mr Micawber and CJ. I quite understand the difference now. (y)

Kind regards,
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