Why does a formal letter carry a subject line, and what makes a good one?

Write a letter to the Editor of a national daily drawing attention to the increasing amount of plastic waste in your city and suggesting measures.

Write the correct subscription for a formal letter beginning "Dear Sir" and for one beginning "Dear Mr Sharma".

Write a letter to the Municipal Commissioner of your city complaining about the poor condition of the roads in your locality and asking for repairs.

Write a letter to your friend describing a book you have recently read and recommending it.

State four differences between a formal and an informal letter.

Write a letter to the Manager of a company complaining about a defective appliance you bought and asking for a replacement or a refund.