Modals - Degrees of Certainty

Modals - Degrees of Certainty:
(1) Present/Future
When we believe that a future state or event is certain to occur, we use will or won't:

			I'll be working on this report all afternoon.
			We won't be back in until tomorrow.

When we deduce that a future state or event is the most logical or rational outcome, we use must or can't/couldn't:

			You must be joking! That's just totally illogical!
			You can't be serious! That's just totally illogical!
			He couldn't be there now, surely. He always leaves at 4.30.

When we want something to happen, and it is reasonable to expect it to, we use should or shouldn't:

			We should be able to go to Spain next holidays if we keep saving at this rate.
			It shouldn't take us long to clean up this mess if we all help.

When we wish to express the something will possibly happen, we use may or may not:

			We may go to the party - we haven't quite decided yet.
			You may not be able to get in if you turn up at the last minute.

When we want to express that something will possibly happen, but we are less certain, we use might/could or mightn't:

			He might be at home, but he usually goes shopping on Saturday morning.
			He could be at home, but he usually goes shopping on Saturday morning.
			She mightn't be able to come - her mother's very ill.

Note that couldn't is not used in this way, but rather expresses a greater degree of certainty (see must or can't/couldn't above).

(2) Past
All of these modal verbs can also be used to talk about degrees of certainty in the past. Once more, will or won't expresses the most certainty, and might/could or mightn't the least certainty.

			That will have been Ted you saw - he's seven feet tall.
			It won't have been Sue you saw - she's blonde and is five feet tall.

Notice that would and wouldn't can be used in the same way as will and would here:

			That would have been Ted you saw - he's seven feet tall.
			It wouldn't have been Sue you saw - she's blonde and is five feet tall.

			That must have been fun - you love dancing, don't you?
			It can't/couldn't have been much fun out on the boat - there were gale-force wind, I hear. (See grammar definitions).

			Where can they be? They should have been here a long time ago.
			Where can they be? They shouldn't have taken this long.

			She may have dropped by - we were out all morning.
			She may not have been able to see properly in the heavy rain.

			He might have had an accident!
			She mightn't have even known we were going to be here.

more info : http://gracielablog-english.blogspot.com/2010/08/modals-degrees-of-certainty.html