Standard business letter is a very formal letter as opposed to informal personal letters. Follow some basic rules to write an effective business letter.
Why are you writing? Say it concisely and clearly A good business letter is more than a narrative; it should supply all important information in a concise and clear way. Write your business letter in a standard format with 1” margin and serif font such as Arial (11 point) or Times New Roman (12 point), with sentences of 2.5 lines or shorter length. Conventional business letter includes the inside address, the date and salutation along with the letter content. Use your words wisely and show that you are on a limited budget for every word you use.

Explain your purpose of writing in the beginning sentence. Most of the business professionals generally skim through their received business correspondence. So, it is necessary to state your reason of writing this business letter in the opening sentence. Use straight language with use of active voice instead of passive voice.
Write with a formal tone

Write your business letter in a more formal letter and keep away using conversational English. However your letter must sound natural keeping your reader in mind. Make good use of your words so that your audience slow down to read the letter word by word after first usual skimming.
Keep it free of any grammatical or spelling errors Make no spelling or grammar mistake – not a single one. When you are equipped with latest word-processing software this is a no-excuse matter.