The former has all entries tight against the left -hand margin. The semi-blocked format sets the references and the date to the right margin for filing and retrieval purposes, with the remaining entries placed against the left margin. The indented format follows the same layout as either of the above, but indents each paragraph by five or six spaces. Clarity of communication is the primary goal. Don't use technical jargon if the recipient is unlikely to understand it.
Short sentences are less likely to be misunderstood or misinterperted.
Be precise , don't ramble. Check each sentence to see if it is relevant. Always try to personalize your letters. Always try to be civil and friendly even if the subject matter is stern and sensitive. Give the impression to the recipient that some effort and thought has gone into the letter.