Hi, Alexander!  Recently you wrote in a message to Ardith Hinton:

AK>  I went to school to meet with headmaster. Capital of the 
AK>  US is Washington. Composer of "Moon sonata" is Beethoven. 
AK>  After school I went home. Head is part of body.

AK>  Are there any examples how to disprove my rule? ;-)


          After school [or work] I went home... that's correct.


          WRT your other examples here, the article may be needed when you are
*not* using nouns in apposition... as you did in your initial enquiry.
  
            The head is part of the body.

            The capital of Canada is Ottawa, Ontario.

            The President of the US remarked yesterday that... [blah blah].

            I have an appointment at the eye clinic.


          but:

  
            Leo Tolstoy, [the] author of WAR AND PEACE

            Joe Biden, [the] President of the US

            Philip, Duke of Edinburgh

            George Gordon, Lord Byron


          In general the article may be omitted when some individual's name is mentioned first & a bit of clarification is added almost immediately... either separated by a comma, as above, or at times by the verb "[to be]":
  
            Justin Trudeau is [the] Prime Minister of Canada.

            Susan Smith is [the] General Manager of the XYZ Company.

       
          There's always a possibility... however remote... that more than one person uses the same name &/or that a member of the audience will have no idea who you're talking about until you've added a bit of further information.  Now
... if what I've said is as clear as mud, please feel free to ask again.  :-))




--- timEd/386 1.10.y2k+