I'm spanish and I have a problem, with the best choice to the modal must and may for a sentence. According my book two both express possibility. Then Could you help me with the best choice of the modal verb for the next sentence?

Debe haberse quedado dormido.
he may have overslept.
he must have overslept.

Why your choice?


From talkenglish:


  • Used to ask for formal permission.

  • Used to suggest something that is possible.


  • Used to express something formally required or necessary.
  • Used to show that something is very likely.

See also:

  • According talkenglish neither "may" nor "must" could be used. but according my lecture it is posible. Who is a own of the truth? my book or talkenglish? – juaninf Mar 30 '15 at 11:26
  • 2
    "He may have overslept." It is possible that he slept too long, but maybe not. "He must have overslept." I didn't personally witness it, but I conclude that it is likely that he slept too long. You might say either sentence if a person was late for an early-morning appointment, for example. – Jay Mar 30 '15 at 13:28
  • 2
    According to talkenglish both CAN be used. Use the second definition for each word. "May" means it's possible, but you're not sure. "Must" means it's possible, and you think it's very likely. Which word is best depends on how sure you are. Use "may" if you are 50% sure he overslept. Use "must" if you are 85% sure he overslept. – Keiki Mar 30 '15 at 16:32

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