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.

(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.

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 obligationI 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 obligationYou 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.

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).

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.

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.

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.

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.

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!

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.

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?

http://www.learnamericanenglishonline.com/Green%20Level/G8%20Modal%20Passive.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/