* Recipient: Who are you writing to? A formal letter is appropriate for official communications with government agencies, businesses, professional colleagues you don't know well, or anyone requiring a highly respectful and professional tone. An informal letter is better for friends, family, and close colleagues.
* Purpose: What is the letter's goal? Formal letters are best for making official requests, complaints, applications, or conveying important information that requires a documented record. Informal letters are suitable for casual communication, sharing personal news, or maintaining personal relationships.
* Relationship: What is your relationship with the recipient? A formal tone is usually required for strangers or those in positions of authority. An informal tone is acceptable for those you know well.
* Content: Is the subject matter serious or sensitive? Formal language is generally preferred for sensitive issues or situations requiring professionalism and clarity.
In short, there's no single answer. Assess the situation and choose the level of formality that best suits the circumstances.