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 Making Sense of The Modals: Part 2  
That might be Martha.
That may be Martha.
That should be Martha.
That must be Martha.
Other uses of modals:
There are four other uses of modals that are somewhat different from the uses described above. They are:
Ability-can, be able to:  
I can speak Spanish.
Maria is able to run fast.          
Desire-would like to:
Sarah would like to visit London.         
Offer-would you like...?
Would you like anything to drink?         
Preference-would rather, would prefer to:
I would rather study languages than math.
Martha would prefer to go to school instead of working.  
Additional information:
There are some more facts about modals that we should observe:
1- the negation of a modal (meaning to use a modal in the negative form)
To use a modal in a negative sentence, we put 'not' after the modal, for example: I cannot do it
I am not able to do it 
We will not eat at that restaurant.
We are not going to eat at that restaurant.
You should not smoke here.
You are not supposed to smoke here.
Notice that pair of sentences above shares the same meaning.  However, the following is different:
You must not go.
You do not have to go.
The meaning is clearly different.  While the first is a command, the second is not.  It is more of a choice.  
Making sense of it all:  
Now let me try to help you make sense out of this. Modals are words that express possibility, authority, advice or ability.  The level of certainty and/or the level of ability control which modal to use.  Let us look at this:
   Possibility and/or ability
Should advice
Must    command  
Let us review the following sentences and notice the degree of possibility decreases as we use the past for of the modal:
I may visit you tonight.   (50% chance that I will)
I might visit you tonight. (Less than 50% chance I will)
I can run fast.  (I am positively sure I can)
I could run fast.          (I am not 100% sure - possibility may depend on something else)  
Will you shut the door?    (You are close to the door and the possibility of you shutting it is high)
Would you shut the door?   (You are far from the door and you might not want to go shut it)         
You should study harder.   (I am advising you without authority)
You must study harder.     (I am commanding you with authority) 
Modals 3