A and The, a man and mankind

A and The

charity - means a generic term, a type of activity, the concept of charity, as opposed to profit making e.g. I give to charity but not to big business. 

‘A' charity is one charity, such as Oxfam. Oxfam is a charity. Like a football team. 

'The' means the one I mentioned previously, or the important one. 

We live in ‘a' united kingdom, in my opinion, although perhaps our Welsh and Scottish friends would disagree. We live in ‘The' United Kingdom. Capitals for an official name, rather than a type or concept. Use ‘The' because it is THE UNITED KINGDOM OF GREAT BRITAIN etc.   

‘a' good and relevant charity (meaning a or one company).

change 'nearly been to all’   (nearly before been modifies the verb to go/be/ which sounds like you were at the airport and turned back) to   'been to nearly all’

A Man

a man is one man.

A man landed on the moon.

A man - one of many.

The moon - only one moon, as far as we are concerned. Many moons ago. Many moons. But the moon where the man landed. 

Think of it as THE VIP.

So in a short story the first mention of an unnamed man or animal could be a.

A man walked in with a cat. The lady receptionist (the meaning only one receptionist, the VIP here - asking our man questions) asked the man's name. Now the man is called the man because we have mentioned him before and the fact we have described him twice shows he is becoming important.

   'My name is Whittington. Dick.'

    Yes, he's VIP hero of our story, Dick Whittingon, Lord Mayor of London. He was the lord mayor. (Only one at that time.)

   You might use the word a before you know the name of a person. Or to show you know nothing else about them. 

  For example, a Mr John Smith came in.