Either word can be used as an adverb to introduce an expression of regret, though conservatives prefer “regrettably” in sentences like “Regrettably, it rained on the 4th of July.” Within the body of a sentence, however, “regretfully” may be used only to describe the manner in which someone does something: “John had to regretfully decline his beloved’s invitation to go hang-gliding because he was terrified of heights.” If no specified person in the sentence is doing the regretting, but the speaker is simply asserting “it is to be regretted,” the word is “regrettably”: “Their boss is regrettably stubborn.”
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