[Grammar] it must/will/would be very hot

[jutfrank]

I thought they were calling their dad on his cell/mobile phone. Some people switch off their cell phones while at work and use office phones instead.;-)

Okay, fine. This shows how important it is to get the examples that we're working with clear.

 

[NAL123]

The speaker means he can't think of a reason that his dad wouldn't be there. It's still possible he's somewhere else.

Why is the underlined part above correct but not the underlined part below?

It takes just half an hour to reach my dad's office, and he left almost an hour ago. He would be there by now.

 

[GoesStation]

Well, I see what you mean but not really. The use of must shows certainty in the speaker's mind.,

It's frequently used in American English to express not certainty, but doubt that another option is possible. If you were certain you'd just say "He's there." If you can't understand why he might be somewhere else, but you can't absolutely rule out the possibility, you say "He must be there." You can replace "must" with "has to be" or "has got to be" with little or no change in meaning.

 

[jutfrank]

It's frequently used in American English to express not certainty, but doubt that another option is possible. If you were certain you'd just say "He's there." If you can't understand why he might be somewhere else, but you can't absolutely rule out the possibility, you say "He must be there." You can replace "must" with "has to be" or "has got to be" with little or no change in meaning.

It's the same in all varieties of English.

However, as I've suggested, I disagree with what you say above. Without wanting to argue the point too forcefully, let me try to say how:

If there is any doubt in the speaker's mind, it will be expressed by the manner of expression, not by the modal must. The use of must is to make a logical conclusion—I count that as certainty in the linguistic sense we're talking about here.

I get what you mean though, GS—we often make declarations of certainty while at the same time doubting our own reason/senses/memory/sanity, etc. We say we're sure about something when we have a niggle at the back of our mind that we're not.