They must arrive tomorrow

Question

Are these sentences correct:1-If they left yesterday, they must arrive tomorrow.2-If they left yesterday, they have to arrive tomorrow.3-If they left yesterday, they should arrive tomorrow.They are supposed to express an inference. If they have left yesterday, then it is (highly) likely that they will arrive tomorrow,Gratefully,Navi.

CalifJim · Accepted Answer

All three are correct, but only the last one seems really idiomatic to me. Obligation (as shown by expressions like "must" and "have to") is seldom conditional on any other factors (such as whether "they left yesterday").

Here are the modals that make the most sense to me in that context:

If they left yesterday, they [will / may / might / could / should] arrive tomorrow.

will shows speaker confidence.
may, might, and could show logical possibility.
should shows speaker expectation.

CJ

Navitasan · Answer

Thank you CalifJim for all your kind replies,

So "have to" and "must" necessarily imply obligation and not liklihood in these contexts?

One would not say:

4-If the letter was sent yesterday, it must arrive tomorrow.
or:
5-If the letter was sent yesterday, it has to arrive tomorrow.

Meaning: it is highly likely that it will arrive tomorrow (and obviously not "it is obliged to arrive tomorrow").

Gratefully,
Navi.

CalifJim · Answer

navitasanSo "have to" and "must" necessarily imply obligation and not likelihood in these contexts?
... obviously not "it is obliged to arrive tomorrow"

I completely understand the question, but to be honest, because of the if-clause, neither interpretation springs to mind immediately for a native speaker. One is left wondering what is meant at all.

I would not call that function of must (or have to) "likelihood". It's more like "logical necessity".

There is a problem with the combination of a simple modal tense (must arrive) and the word "tomorrow" (or other adverbs of futurity) when you are conveying the idea of logical necessity. The same applies to the semi-modal "have to" (has to arrive).

For example, these will all be interpreted as related to obligation. (It doesn't matter whether you use must or have to.)

The package must arrive tomorrow.
The team must win the match next week.
Susan has to pay the bill by next Friday.

All the if-clauses in the world prefixed to those statements cannot change that interpretation.
__________

The only thing you can do to save it is to change to the continuous form like this.

It must be arriving tomorrow. (Here you are trying to say It must be going to arrive tomorrow without saying it in such an awkward way.)

Though possible, this pattern doesn't work as well with "have to":

It has to be arriving tomorrow.

Even so, it still sounds a little strange to my ear with the if-clause.

If the letter was sent yesterday, it must be arriving tomorrow. (???)
__________

With logical necessity you don't want if; you want because or therefore (or so).

The letter must be arriving tomorrow because it was sent yesterday.
The letter was sent yesterday, so it must be arriving tomorrow.

You see, it's not a matter of imagining the letter was sent (which is what if implies). It has to be a matter of knowing that the letter was sent so that you can form a logical conclusion about that fact.

CJ

Navitasan · Answer

Thank you very much CalifJim,

I have one more question here:

Could "should" be used in such cases not to imply logical necessity or likelihood, but obligation.

Is this sentence correct:
A-If you have received our letter, you should reply by next week.

Gratefully,
Navi.

CalifJim · Answer

navitasanIs this sentence correct:A-If you have received our letter, you should reply by next week.

It's correct, and it is much more likely to be interpreted as obligation than expectation (logical likelihood).

Isolated sentences with 'should' are often ambiguous.

He should reply by next week.
1. I expect him to reply by next week.
2. It is advisable for him to reply by next week.

CJ

They must arrive tomorrow

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