Objective: Depending on the task.
Intended audience: Depending on the task
Style and register: formal or informal. Depending on the task
Structure:
Greeting
Formal: Dear Sir/Madam, Dear Mr./Mrs. (last name)
Informal: Dear (name), Hi (name)
Introduction
Clearly state the reason you are writing. No details are needed at this point. One sentence will do it.
Body
Give detail of the reason you stated in the previous paragraph.
Final paragraph
Summarize what you expressed and wish for something to be done.
Signature
LETTER SAMPLES
How to write a friendly letter!
How To Write Emails In English
How to Write an Email in English - Business English Writing