should/must have completed

[joham]

He _____ have completed his work; otherwise he wouldn't be enjoying himself by the seaside.
The answer is 'must'. I was wondering if we could also use 'should'.
Might I ask native English teachers to help me? Thank you very much.

 

[Tdol]

'Must' makes better sense to me because it is an assumption based on the current situation.

 

[riverkid]

See,

https://www.usingenglish.com/forum/ask-teacher/61155-shouldn-t-couldn-t.html

 

[susanleekl]

Ah.,. this is useful. Thanks.

 

[riverkid]

He _____ have completed his work; otherwise he wouldn't be enjoying himself by the seaside.
The answer is 'must'. I was wondering if we could also use 'should'.
Might I ask native English teachers to help me? Thank you very much.

Here, 'should' feels like it only has a deontic/social modal meaning, as in,

He should have completed his work; he shouldn't be enjoying himself by the seaside when there's still so much to be done.