What does it mean by you are of a great family? When do you use ‘of’ after ‘is’? Thanks in advance.
Hi Clark, what is the phrase or sentence you don’t understand? Maybe, you mean ‘you are of a great family’ = ‘you belong to a great family’ = ‘you are a member of a great family’. At least that’s what I think you mean, no?
Sorry, I didn’t make it clear. I should alter ‘is’ to ‘are’ in this case.
Yes you are right, I did mean something simliar to that, but under what circumstance can you use ‘of’ straight after ‘are’ or ‘is’? 
There are some fixed expressions such as ‘if you are of legal drinking age’ or ‘you are of value’ or ‘you are of Chinese descent’.
Let’s see what Alan has to say on this subject…[YSaerTTEW443543]
TOEIC listening, photographs: Machu Picchu, Peru[YSaerTTEW443543]
i am master of a math :DDD lol, i just kidding 
but i love math
English is of great importance!
You all have presented several different uses of ‘of’:
you are of a great family-- (used to indicate derivation, origin, or source): a man of good family; the plays of Shakespeare; a piece of cake.
you are of legal drinking age-- (used to indicate inclusion in a number, class, or whole): one of us.
you are of value-- (used to indicate qualities or attributes): an ambassador of remarkable tact
you are of Chinese descent-- (used to indicate derivation, origin, or source): a man of good family; the plays of Shakespeare; a piece of cake.
I am master of math-- (used to indicate the objective relation, the object of the action noted by the preceding noun or the application of a verb or adjective): the ringing of bells; He writes her of home; I’m tired of working.
English is of great importance-- (used to indicate qualities or attributes): an ambassador of remarkable tact.
Many of the many concepts of ‘of’ overlap, so that sometimes more than one interpretation is possible.