The negative form of should is should
not (which is
to shouldn't). Shouldn't
also be used to show advisability:
I shouldn't surf the
Internet tonight because I have
a lot of homework
(It wouldn't be a good idea
for me to surf the Internet
tonight because I
have a lot of homework to do, but
whether I surf
the Internet or not is up to me.)
You look tired. You shouldn't work so hard.
(Because you look tired, I
think you're working too hard.
I also think it
would be advisable for you not to work as
as you do. Whether you work hard or not is up to
No, you shouldn't tell the teacher that X was cheating
the test because you're not really sure that he was.
(I think that because you're
not sure that X was cheating
on the test, it wouldn't
be a good idea for you to tell the
he was--but whether you tell the teacher or not
up to you.)
Should does not
show a requirement.
When should is used
to show advisability,
something is a good idea--but there is
a choice about whether or not to do it.