Be able to instead of can
Be able to often has the same meaning as can.
He can walk on his hands. OR He is able to walk on his hands.
I am unable to understand his motive. OR
I can’t understand his motive. 
They were able to catch the thief. OR They could catch the thief.
They were able to catch the thief. OR They could catch the thief.
Be to instead of will or shall
The structure be + to can be used to express simple
futurity.
He is to retire next year. = He will retire next year.
The
President is to visit Japan next
month. = The President will visit
Japan next month. We are to get a wage rise in June. = We will get a wage rise in June.
We are to get a wage rise in June. = We will get a wage rise in June.
Be + to instead of must
The structure be + to is also used to give orders. In
this case, it means almost like must.
You are to complete the work in two days.
(= You must complete the work in two days.) He is to report for duty within a week.
He is to report for duty within a week.
Had better instead of should or ought to
You had better consult a good doctor. = You
should consult a good doctor.
Had better may also express a threat.
He had better be careful.
Have to instead of must
I have to be there at 10 o’clock. (= I
must be there at 10 o’clock.)
