What is the difference between “to sleep” and ” to fall asleep”? and when is used each one?
To sleep means that you go to sleep for a nap or for the night. You can sleep for 8 hours, for example. You can already be asleep, and sleep in an airport or a hotel or your own bed. This is the generic verb.
To fall asleep is much more specific. It means that you go from being awake to being asleep – right when you go to sleep. It refers only to that time when you change from wakefulness to being asleep. You can think of it as the transition between those two states.