CAN
Can is used to express Ability,permission & possibility.
For Example:
1.I can do the sum (Ability)
2.Can i use your pen?(permission)
3.It can be the best of all (possibility)
COULD
A. Could is the past from of 'Can' which is expressed permanent Ability of Past .
For Example:
1.He could work 15 hours a day.
B.In past we use 'was/were able to 'or manage to' instead of 'can/could' when we only a work had done.
For Example:
1.He managed to pass the test.(NOT he could pass the test.)
2.He was able to pass the test
3.He could not pass the test.
C.Could is also used to request
For Example:
1. I have a headache .could you buy some aspirine for me?
MAY/MIGHT
A.may/might is used to mean possibility
For Example:
1.It may rain today(may be)
2.It maight rain today(it will possible)
B.May/Might also used to permission
For Example:
1.May i come in sir
C. You guess something in present
For Example:
1. They may/might be watching tv (may/might be + ing) present continuous
D.You guess something in past (may/might have +v.p.p)
For example:
1.You might have heard about Milton
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