fall asleep 表示“入睡， 往往含有”不想入睡而睡着了“或”不知不觉地睡着了“，be asleep 表示”睡着，熟睡“的状态go to sleep.指”入睡，进入梦乡“这个动作，强调动作的结果。Soon he went to sleep for he was very tired.He was asleep with his head on his arms.One after another, all three of them fell asleep