Changing the mood of a sentence (modals)

In English, you can show what you feel about a situation by using words such as may, will, would, might, can and could.These words can change the meaning of a sentence and show that something is possible, necessary, uncertain, or intended.
"For example, "I'll go shopping tomorrow" shows that you intend to go tomorrow.
"I might go shopping tomorrow" shows that perhaps you will go tomorrow, but you don't know for sure.
When you use these words, remember:
* They are followed by the verb without to.
* You don't need an 's' for the third person singular: "He might come to the party." (Not "he mights come to the party".)
* You can make a question by putting the word before the person, not by using 'do' or 'have': "Could you help me?" (Not "do you could help me?")
* You can make a negative form by adding a form of not to the word:
can becomes can't
will becomes won't
might becomes might not (or mightn't)may becomes may not
could becomes couldn't.
How possible something is
"The company might relocate next year."
"We may have to wait an hour for dinner in this restaurant."
"It can get very cold here in winter."
"We could all live to be 100 years old in theory."
How certain something is
"She'll get promoted next year."
"He won't agree to that idea."
"You must be our new neighbour."
"If you left now, you would get the train."
Offers and requests
"Shall I open the door for you?"
"I'll cook dinner, if you like."
"Could you help me?"
"Can you pass me the salt, please?"
Permission
"Can I open the window, please?"
"You may now look at your exam papers." (This is formal.)
Ability
"I can cook, but I can't drive."
"I couldn't speak French very well when I was at school."
Using should, must and need
These words help you to talk about rules, obligation and advice.
Should
We use should to give advice.
"If you want to learn English, you should practise as much as possible."
We can also use should to talk about what we expect to happen.
"He should be here by now – he left over an hour ago."
The negative of should is shouldn't.
"You shouldn't eat so much chocolate – it's bad for you."
Must
We use must to talk about obligation.
"I must call my grandmother today – it's her birthday."
If you want to say the opposite – that there is no obligation to do something, use don't have to or don't need to.
"You don't have to wash the car – I'll do it."
"You don't need to put the rubbish out – I've already done it."
We can also use must to talk about what we think is logically certain.
"You must be tired after all that travelling."
If you want to say that something is logically impossible, use can't.
"Who's that at the door? It can't be the postman – he's already been."
Mustn't means that it is not allowed to do something.
"You mustn't feed animals in the zoo – it's not allowed."
Should have done
Look at this example dialogue:
"You know… my car was broken into yesterday."
"How terrible. What did you do?"
"Oh, nothing."
"You should have called the police."
We use the pattern should have done to talk about what we think would have been the best thing to do. However, the past event we are talking about cannot be changed. So the pattern after should is have done – not should do.
In the example dialogue, the person didn't contact the police yesterday (in the past), so you can't change the situation. You can only say what action would have been the best in this situation