Semi-formal letter is type of the letter usually being sent to people we do not know very well or in situations which require more polite and respectful approach (e.g. a schoolteacher, school principal, etc.).
Thus, these letters are written in a more polite tone than informal letters. Here are some tips of what a semi-formal letter should consist of:
1. a formal salutation
e.g. dear Mr. and Mrs. Baker, use always last name since you know recipient, do not start with Dear Sir, Dear Madam.
2. Introduction paragraph
Here you should state your opening remarks and clearly state the reason(purpose, motivation) for writing
e.g. Thank you very much indeed for you kind offer…
I am writing you because……
3. Semi-formal letter body
In this paragraph you write the main subject(s) of the letter in detail, starting a new paragraph for each topic
4. A conclusion
This should be a last paragraph of your letter, and you should write your closing remarks
e.g. I am looking forward to seeing you next month…
e.g. I hope you can meet my request (inquiry)….
5 a semi-formal ending
e.g. Regards/Best regards / Best wishes, etc. + your full name
Other tips: less frequent use of short forms, phrasal verbs and idioms; use a polite, respectful tone.