Like the rules governing the use of shall
on which they are based, the traditional rules governing the use of should
are largely ignored in modern American practice. Either should
can now be used in the first person to express conditional futurity: If I had known that, I would (or somewhat more formally, should) have answered differently.
But in the second and third persons only would
is used: If he had known that, he would (not should) have answered differently. Would
cannot always be substituted for should,
is used in all three persons in a conditional clause: if I (or you or he) should decide to go. Should
is also used in all three persons to express duty or obligation (the equivalent of ought to
): I (or you or he) should go.
On the other hand, would
is used to express volition or promise: I agreed that I would do it.
is possible as an auxiliary with like, be inclined, be glad, prefer,
and related verbs: I would (or should) like to call your attention to an oversight.
was acceptable on all levels to a large majority of the Usage Panel in an earlier survey and is more common in American usage than should.
· Should have
is sometimes incorrectly written should of
by writers who have mistaken the source of the spoken contraction should've.
See Usage Notes at if