fall asleep强调入睡的动作。
be asleep 强调睡着的状态。
be asleep：睡着了;入睡;睡着;睡着的;睡着了的；
fall asleep：入睡;睡着;进入梦乡。
Try real hard to be asleep by the time I get back, huh?
我回来的时候你最好是睡着了，好吗？
I thought you'd be asleep by now.
我觉得你可以睡一会了。
You're sleepy all day, but when it's time for bed, you can hardly fall asleep.
你一整天都很困,但到了睡觉的时候却睡不着。
He couldn't fall asleep due to the insupportable pain.
由于疼痛难忍，他无法入睡。
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