Originally Posted by
benbaaa
Michael Lewis says in The English Verb that the use of the past simple denotes remoteness. Usually it's remoteness in time (as in I shat a big one yesterday morning), but it can also be social remoteness (which is where overly polite hotel receptionists ask you "What name was it, sir?") or remoteness from reality (which is why it's used in unreal conditionals, such as "If I were you..."). In the other example ("I would imagine he knew by now"), the speaker is distancing himself from the possibility of being wrong - another form of remoteness.
*do not under any circumstances attempt to explain this to Thai learners*
:)