When to use regretfully in a letter?

Use "regretfully" in a letter when you need to convey disappointment or sadness about a situation, such as declining an invitation, expressing sympathy, or informing someone of an unfortunate decision. It adds a tone of sincerity and empathy, making it clear that you acknowledge the impact of the message on the recipient. For example, you might say, "I regretfully must decline your invitation due to prior commitments." This helps to soften the message while maintaining professionalism.