I understand that "to be bound to" is a semi-auxiliary verb but I don`t know its meaning. Although I tried searching it in some dictionaries, I didn`t find its exact meaning. Could you, please, help me with that?
e.g. His holiday is over; he has no other choice so he is bound to come back this evening.
Thank you very much in advance.
"bound" in this phrase means certain and is the past participle of To bind, used as an adjective and usually followed by an infinitive.
I beeeelieve that this collocation occupies the same range of certainty as a 'very likely' to an 'almost certainly'.
Of course, 'almost certainly' occupies the same range as a 'must' but I think that this is a good example of a situation where a 'must' of certainty could not be used. 'must' is ['almost certainly' + deductive reasoning] that leaves the speaker only one rational choice.
For pure speculation without the "facts that provide the fodder for deductive reasoning, we'd choose 'almost certainly' or in the 'should' range, 'likely/probably' 'very likely'.