+170.+ _Should_ and _would_ are the past-time forms of _shall_ and
_will_. We use them to express action or existence dependent upon some
condition, thus:
I should go if I were well enough.
He should join us if you asked him.
In these sentences _should_ and _would_ express action which is possible
now or will be in the future, provided some other action takes place.
The same distinction which we found made in the use _shall_ and _will_
has been made with _should_ and _would_; that is, that _should_ used
with the first person, expresses action dependent upon condition; but
_would_, used with the first person, implies exercise of the will. This
rule is not closely followed, though it expresses a nice distinction in
the use of _should_ and _would_. In ordinary usage we use either
_should_ or _would_ with the first person without any distinction of
meaning, as for example:
I should struggle on even if it meant death.
I would stand for my principles though I stood entirely alone.
We do not use _should_ however, with the second and third persons to
express an action or existence dependent upon some condition. _Should_
used with the second and third person implies obligation. _Would_ is
used with the second or third person to express an action dependent upon
some condition, as for example:
He would not go, even if you insisted.
They would come if you invited them.
You would believe him if you could hear him.
You would be surprised if I should tell you the reason.
+171.+ _Should_ and _would_ in all of the sentences which we have quoted
are used to express action or existence dependent upon some condition
which is expressed in that part of the sentence introduced by such
conjunctions as _if_ and _though_.
The parts of the sentence introduced by these conjunctions express the
condition upon which the other action is dependent. When we use _should_
in sentences without this condition, it means practically the same as
_ought_, and implies an obligation. We use _should_ with the first and
second and third persons when we use it with this meaning, as for
example:
I should have gone yesterday.
You should be with us in this fight.
They should never fear defeat.
+172.+ _Ought_ could be used in all these sentences and express
practically the same meaning. _Should_ used in this way implies
obligation.