Теория:

Rules for writing formal letters. Business communication.
  
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You should try to write as simply and as clearly as possible, and not to make the letter longer than necessary. Remember not to use informal language.
 
Addresses.
 
1. Your address:
The return address should be written in the top right-hand corner of the letter.
 
2. The address of the person you are writing to:
The inside address should be written on the left, starting below your address.
 
3. Date:
Different people put the date on different sides of the page. You can write this on the right or the left on the line after the address you are writing to. Write the month as a word.
 
Salutation or greeting.
 
1. Dear Sir or Madam,
If you do not know the name of the person you are writing to, use this. It is always advisable to try to find out a name.
 
2. Dear Mr Jenkins,
If you know the name, use the title (Mr, Mrs, Miss or Ms, Dr, etc.) and the surname only. If you are writing to a woman and do not know if she uses Mrs or Miss, you can use Ms, which is for married and single women.
 
Ending a letter.
  
Use different formal phrases:
  
1. Yours faithfully,
If you do not know the name of the person, end the letter this way.
 
2. Yours sincerely,
If you know the name of the person, end the letter this way.
 
3. Your signature:
Sign your name, then print it underneath the signature.
Источники:
http://www.nvtc.ee/
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