The English modal verbs are often challenging for learners
of English. This happens for many reasons, including both
grammar and meaning.

In this Hint, we'll take our first look at should.

The modal auxiliary should has several uses. The most
common one is probably in showing advisability--that is,
in showing that something is a good idea. It's important
to understand that when should is used in this way, there is
always a choice for whether to do something or not--
because advisability is not the same as a requirement.

Examples:

I should study tonight.

(It would be a good idea for me to study tonight, but
maybe I'll study and maybe I won't.)

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You look tired. You should rest.

(Because you look tired, I think it would be advisable
for you to rest--but I know that maybe you'll rest and
maybe you won't.)

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I think X was cheating on the test. Should I tell
the teacher?

(I think, but don't know, that X was cheating on the test.
Do you think it would be a good idea for me to tell the
teacher? Maybe I'll tell the teacher and maybe I won't,
but I want to know what you think would be advisable.)

The negative form of should is should not (which is
commonly contracted to shouldn't). Shouldn't can
also be used to show advisability:

Examples:

I shouldn't surf the Internet tonight because I have
a lot of homework to do.

(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.)

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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
hard as you do. Whether you work hard or not is up to
you, however.)

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No, you shouldn't tell the teacher that X was cheating
on 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
teacher that he was--but whether you tell the teacher or not
is up to you.)

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Special Note:

Should does not show a requirement. When should is used
to show advisability, something is a good idea--but there is
always a choice about whether or not to do it.