should and should have, would and would have, could and could have
Danilo
Gomez Barbosa from Columbia writes:
Could
you please explain the difference between the modal auxiliary verbs
should, could and would and how they are used? Thanks
for your help.
Asim
from Pakistan writes:
I still
feel some complication in understanding these modals: would have,
should have and could have. Please give me some examples
to help me understand.
Roger
Woodham replies:
should
Should is used to give advice and make recommendations and
to talk about obligation, duty and what is expected to happen. Reference
is to the present and the future. Should is similar to must
but is not as strong as must:
You should always wear a helmet when
you go out cycling on busy roads.
Once the pack is opened, the cooked meat inside shouldbeconsumed within three days.
Should I tell her that her son is playing truant and
skipping school? ~ I think you should. She should know
about it.
should
and should have
Should combines with the perfect infinitive to form should have
+ past participle when we want to talk about past events that
did not happen, but should have happened. We are talking
about an expectation and referring back to past time. Compare the
following:
Before Tom leaves for work, his wife advises him:
You should take your umbrella. It might rain. ~ No,
I'll be all right. I shan't need it.
But it did rain. When he arrives back home, his wife says:
What did I tell you? You should have taken your umbrella.
Then you wouldn't have got wet.
Reference to the present and future:
You should try and smoke less, Henry. Your
health isn't very good and it's getting worse.
Reference to the past:
I should have given up smoking years ago, Mary. If
I had, I wouldn't be in such bad shape now.
would
If we want to talk about an unreal or unlikely situation that might
arise now or in the future, we use a past tense in the if-clause
and would + infinitive in the main clause. Compare
the following and note that would is often abbreviated to
'd:
How would you manage, if I wasn't here to help you?
~ I'd manage somehow. I wouldn't bother to cook.
I'd go out to eat or bring home a take-away. I'd ask
your mother to help me with the washing and the ironing. I know
she'd help me.
would have
If we want to refer to the past and make a statement about things
that did not happen, we need to use had + past participle in
the if clause and would have constructions in the
main clause. Note in these sentences that we can use 'd as
the abbreviation for both had in the if-clause and would
in the main clause:
If he'd taken an umbrella, he wouldn't have got wet on
the way home.
If he'd taken his umbrella, he'd have stayed dry.
could
Could can be used to ask for permission, to make a request
and express ability in the past. Compare the following:
Could I borrow your black dress for the formal dinner
tomorrow? ~ Of course you can!
Could you do me a favour and pick Pete up from the
station? ~ Of course I will!
I could already swim by the time I was three.
~ Could you really? I couldn't swim until I was eight.
could have
As with would have, and should have, could have is
used to talk about the past and refers to things that people could
have done in the past, but didn't attempt to do or succeed in doing:
I could have goneto university, if I'd passed my exams.
If he'd trained harder, I'm sure he could havecompleted
the swim.
Note the difference between would have and could have
in the following two examples. Would have indicates certainty
that he would have won if he had tried harder, could have
indicates that it is a possibility. Might have is similar
in meaning to could have, although the possibility is perhaps
not quite as great:
If
he'd tried a bit harder, he would have won the race.
If
he'd tried a bit harder, he could have won the race.
If
he'd tried a bit harder, he might have won the race.
should have / could have / wouldn't have
Note the way in which all three of these modals are combined in
these exchanges which refer to a meeting that has just taken place:
Why did you come to the meeting? It didn't need both of us.
You should have known that I would be there. ~ How could
I have known you'd be there? I haven't spoken to you for a
fortnight! ~ If I'd known you were intending to go, I certainly
wouldn't have gone!
You will sometimes see wouldhave written as would've,
shouldhave as should've and couldhave
as could've.
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