Modal auxiliary verb of probalbility.

 

Modal Auxiliary Verb of probability

The main modal auxiliary verbs that express probability are described here in order of certainty.

WILL
Will and won't are used to predict a future action. the truth or certainty of what is asserted
is more or less taken for granted.

I'll see you later.
His latest book will be out next month.

MUST AND CAN'T
Must is used to assert what we infer or conclude to be the most logical or rational interpretation of a situation. We do not have all facts, so it is less certain than will.


You say he walked across the Sahara Desert! He must be crazy!
You must be joking! I simply don't believe you.  

Can't is use in negative form

She can't have a ten-years-old daughter. She's only 21 herself.

SHOULD
Should expresses what may reasonably be expected to happened. This use of should has the idea of if everything has gone according to plan.

Our guests should be here soon (if they haven't gotten lose)
This homework shouldn't take you too long( if you've understood  what you have to do)

Should in this use has the idea that we want the action to happen. It is not used to express negative or unpleasant ideas.

You should pass the exam. You've worked hard
You should fail the exam INCORRECT

MAY AND MIGHT
May expresses the possibility that and event will happen or is happening.

We may go to Greece this year. We haven't decided yet.
Where is Ann? She may be taking a bath, I don't know.

Might is more tentative and slightly less certain than may.

It might rain. Take an umbrella.
Where is Peter? He might be upstairs. There is a light on.

COULD
Could has the similar meaning to might.

You could be right. I'm not sure.
That movie could be worth seeing. It got good reviews.

Couldn't is not used to express a future possibility. The of could in this use is might not.

You might not be right.
That movie might not be any good.

Couldn't has a similar meaning to can't above, only slightly weaker.

She couldn't have a ten-years-old daughter! She's only 21 herself.



 These links are for to practice.

http://speakspeak.com/english-grammar-exercises/intermediate/intermediate-grammar-exercise-modal-verbs-for-present-probability-must-cant-could-may-might
http://www.autoenglish.org/gr.may3.i.htm