Can, could, may, might, must, should, will, shall, would
Modal verbs convey a subjective concept often expressing the judgment, intention, point of view and role of the speaker. Learning how to use them will save you in many embarrassing situations when you don’t really know how to express an idea. Here is why I believe you should use modal verbs in speaking:
Of course, everything has pros must have some cons too. Here you have the cons of using modal verbs.
Many languages don’t have an equivalent for English modal verbs and many learners avoid using the modal verbs because they just don’t make sense for them. In fact, using modal verbs is very simple and since they are always followed by a verb in its base form, the grammar used in this kind of sentences should be mistakes free.
The usage of "should" and "shouldn’t"
Use "should" or "shouldn’t" + base form of a verb to give somebody advice or say what you think is the right thing to do.
Note: "Should/shouldn’t" is often used as a form of criticism, especially with the negative.
Example: You shouldn’t drive so fast! (This also depends how the speaker says it – does he sound friendly or as if he’s criticizing.)
I think you should teach for another school.
The government should do more for the country.
She should buy a new motorbike; she always seems to have problems with this one.
She shouldn’t drive so fast.
The two forms (should and ought) are almost always used interchangeably but some consider that "ought" to implies a stronger obligation than "should", while others think that "ought" to is old-fashioned and may always be replaced by "should". And when something sounds old-fashioned, it almost automatically also obtains a more formal register.
The only situation I can think of when "ought" to cannot be replaced with should is in the British English usage of should in the meaning of if:
Should the visitors arrive early, we can have lunch with them. (Not common in American English but often used in British English).
Ought the visitors to arrive early, we can have lunch with them.
The learners of English should remember that "ought" always goes with "to".
This article treats the usage of "may/may not" and "might/might not", two modal verbs that function very similarly.
The rules for using may/may not and might/might not
Use "may/may not" and "might/might not" to talk about future possibilities.
We might make a barbeque in the garden tomorrow, but it depends on the weather.
I might not go to the cinema on Sunday, I haven’t decided yet.
I may go to the party, but I’m not sure.
I may not have time to finish all I have to do today.
"Might" is also used to show the speaker’s irritation:
You might have told me you were coming late; now the dinner is ruined!
"Might" is also used in polite suggestions:
You might want to consider a couple of other points also connected to your plan.
"Might" can also be used for [very] polite inquiries:
Hotel receptionist (speaking to a hotel guest): “Might you be having dinner at the hotel restaurant this evening, sir?” (However, this can be seen as rather archaic by many, and the more common structure is Will + S + be + V+ing…?)
Another approach to introducing modal verbs of probability/likelihood is to show them together in a form of a scale to clarify their differences:
A: Where’s Peter?
B: He can’t be at home./He must be at home.
B: He should be at home.
B: He may be at home.
B: He might/could be at home.
The use of "can’t/must" means that the speaker is, more or less, 100% sure this is true.
(a) "Must" is used in positive statements, and can be paraphrased as [The speaker] is sure that…. In context of the example above: (Example: I’m sure that Peter’s home.) "Can't" is used in negative statements, and can be paraphrased as [The speaker] is sure that [something] is not true. In context of the example above: (Example: I’m sure that Peter is not at home.)
The use of "should" denotes that speaker is fairly sure about what he is saying because he knows this is a regular routine, and can be paraphrased as [The speaker] is fairly sure that [something] is true because he knows the routine behind this activity. In context of the example above: (Example: I am fairly sure that he’s at home because I know he’s usually there at this time of day.)
"May" is used when the speaker has doubts about whether what he/she’s saying is true or not. When [the speaker] comments on the probability of [something], he is actually saying “perhaps this is true, perhaps it’s not”. In context of the example above: (Example: Perhaps he’s at home, but equally well he can be somewhere else.)
"Might" and "could" are used similarly to "may", but usually indicate that there’s a variety of choices, any of which could be true.
The past tense for the modals of probability/likelihood/deduction is formed thus: MOD + Present Perfect Infinitive.
A: Where was Peter yesterday evening?
B: He must have been at home.
He can’t have been at home.
He should have been at home.
He may have been at home.
He might/could have been at home.
A point of warning:
One more clarification must be made: when you use the negative form of "could" (couldn’t), it can be used instead of "can’t", with the meaning "I’m sure that … not …". Therefore, it’s not advisable to use it as an alternative expression to might not.
How to use “have/has to” and “don’t have/has to”
Being a modal verb, “have/has to” is followed by a verb in base form and it says that something is necessary.
I have to ride the bike three hours a day.
My daughter has to study English every day.
To say there is no obligation or something is not necessary, use “don’t/doesn’t have to” followed by base form.
We don’t have to wear uniform at school in Romania.
His wife doesn’t have to work, she is rich.
“Do/does” is used for forming questions and negatives.
Why do you have to study English?
Does she have to work on Sunday?
“Must” is followed by a verb in base form and express strong obligations and rules.
The traffic sign says: “All motorcycles must turn left”.
Use “must not” or “can’t” to say something is prohibited.
You must not leave your luggage unattended in the airport.
You can’t bring food into this cinema.
To form questions you should use “have to” (it is more popular than “must”).
Do I have to buy a ticket?