Tunefully is the adverb from tuneful. It tells you HOW someone sang, played, whistled or hummed: in a way that sounded pleasant, with a clear and agreeable melody. Because it is an adverb, it goes with a verb, not with a noun — "she sang tunefully", not "a tunefully song". That is the whole grammatical difference between this word and tuneful, and it is the thing students mix up most. Where you actually meet it: describing a person singing or playing, in writing rather than in speech. "He whistled tunefully as he worked." "The children sang tunefully enough, considering they had only practised twice." It often appears with a small qualifier like that — quite tunefully, tunefully enough — because it is the kind of moderate praise that likes a little hedging next to it. Like tuneful, it feels literary and a bit old-fashioned; you will find it in a novel or a review much more often than in a conversation. In everyday speech, people simply say "she sang beautifully" or "he sang well". The natural opposite is tunelessly.

（唱、吹、彈得）旋律優美地、很好聽地。是 tuneful 的副詞，用來形容「怎麼唱」而不是形容那首歌。中文一句「她唱得很好聽」就到位了；英文寫成 tunefully 帶點文學味，平常講話會直接說 sang beautifully 或 sang well。