This document provides information and examples about the use of modal verbs and expressions related to obligation, possibility, necessity, and hypothetical or unreal situations in the past. It discusses modal verbs like can, may, must, have to, should, would in both present and past forms. It also covers expressions using if, wish, rather, and it's time to talk about hypothetical, counterfactual, preferred and suitable scenarios.

2.  S + modal + be + past participle + complements
 Children can be taught at home.
 Horses may be ridden in this park.
 People must be warned about swimming in this lake.
 This work has to be done by tomorrow morning.
 Something should be done about global warming.
 A lot of cows can be raised on a productive farm.

3. (past modal verb) + have + been +
past participle + complements
 They could have been made in the oven.
 Earlier, these bikes might have been
taken through the streets of Minneapolis.
 Corn would have been grown last summer.
 This had to have been done by yesterday.
 Something should have been done many
years ago but wasn't.
 Corn could have been raised, but this
farmer decided to grow grass.

4. Present Positive Negative
have to /
don't have to
•strong obligation (possibly from
outside) Children have to go to
school.
(sometimes 'have got to')
•no obligation
I don't have to work on Sundays.
•
You don't have to eat anything you
don't like.
must / mustn't •strong obligation (possibly based on
the speaker's opinion) I must study
today.
•negative obligation
You mustn't smoke here.
should / shouldn't
•mild obligation or advice.
• You should save some money.
•mild negative obligation or advice
You shouldn't smoke so much.

5. Be careful about the difference between mustn't and don't have to!
Mustn't means it's not allowed, or it's a bad idea:
You mustn't eat so much chocolate, you'll be sick
Don't have to means you don't need to do something, but it's fine if you want to do it:
I don't have to get up early at the weekend (of course, if I want to get up early, that's fine,
but I can stay in bed if I want).

6. Past Positive Negative
had to / didn't have to •Obligation in the past.
• I had to wear a school
uniform when I was a child.
•No obligation in the past.
• We didn't have to go to
school on Saturdays.
must* changes to 'had to' -
should have + pp / shouldn't
have + pp
•A past action which didn't
happen: the advice / regret is
too late.
•You should have gone to bed
earlier, now you have missed
the train.
•A past action which didn't
happen: the advice / regret is
too late .
•You shouldn't have taken
that job., it was a bad idea.

7.  The expressions if, supposing, if only, what if can be used to introduce hypothetical
situations and followed by a the simple past tense to indicate that the condition
they introduce is imaginary.
 Examples
 Supposing an elephant and a mouse fell in love.
 What if we painted the room yellow?
 If you went to the movies, I would babysit.
 If only I had more money, I could go to the movies too.

8.  These expressions can also introduce hypothetical situations in the past and then
they are followed by the past perfect.
 Examples
 If only I hadn't kissed the frog.
 What if the elephant had stepped on my phone?
 Supposing I had given that man my money.

9.  The verb to wish is used with the unreal past when we want to talk about situations
in the present that we are not happy about but cannot change.
 Examples
 I wish I had more money.
 She wishes she was beautiful.
 We wish we could come to your party.
 When we want to talk about situations in the past that we are not happy about or
actions that we regret, we use the verb to wish followed by the past perfect.
 Examples
 I wish I hadn't said that.
 He wishes he hadn't bought the car.
 I wish I had taken that job in New York.

10.  When we want to talk about situations we are not happy about and where we want
someone else to change them, we use to wish followed by would + infinitive.
 Examples
 I wish he would stop smoking.
 I wish you would go away.
 I wish you wouldn't squeeze the toothpaste from the middle!

11.  When we want to talk about a course of action we would prefer someone else to
take, we use I'd rather + past tense.
 Examples
 I'd rather you went.
 He'd rather you called the police.
 I'd rather you didn't hunt elephants.

12.  When we want to say that now is a suitable moment to do something, either for
ourselves or for someone else, we use it's time + past tense.
 Examples
 It's time you paid that bill.
 It's time I went home.
 Don't you think it's time you had a haircut?

13.  http://www.learnamericanenglishonline.com/Green%20Level/G8%20Modal%20Passiv
e.html
 http://www.learnamericanenglishonline.com/Green%20Level/G9%20Past%20Modal%
20Passive.html
 http://www.perfect-english-grammar.com/modal-verbs-of-obligation.html
 http://www.edufind.com/english-grammar/unreal-past/