The characteristic of modal verbs is they sometimes have many similar meanings, and they are used in many diffrent contexts.
Generally, we use modals to express ability, possibility, request, permission, deny permission, order, etc.
The English modals are:
can, could, be able to
must, have to
should, ought to
Let's start from a few general rules.
1. You don't have to conjugate them:
2. After modals we use infinitive without to (so called bare infinitive). The only exception is ought to:
3. Modals are not used with auxiliary verbs. In negation we add not and in questions we employ inversion:
4. They can occur as a continuous form: be + verb + -ing:
In negation we use can't (can + not) or cannot:
Talking about the past we use could or be (was/were) able to.
Could informs generally about an ability whereas be able to relates to a specific situation.
We often connect could with such expressions as see, hear, smell, taste, feel, remember, understand:
The expression be able to replaces can and could in all other tenses (except present simple and past simple):
Talking about possibility both forms can and could are allowed (although the past form could decreases degree of probability):
In relation to the past we use could + have + past participle:
Using can't we can express that something isn't possible (because we know that for sure):
Remember that could is more polite than can:
In this case may and might can be used interchangeably; however, might means less probability:
Talking about past we use may/might + have + past participle:
May and might are also used interchangeably:
Talking about unreal situation we only use might:
In this context the continuous form is also possible:
When our intention depends not on us we use may. These sentences are very polite or very formal:
In relation to the perfect, past and future instead of may we use be allowed to (in the past could/couldn't, too):
Talking about the past we employ must + have + past participle or can't + have + past participle:
Must shows personal belief and involvement of the speakers:
Have to does not express personal feelings but is connected with conditions, rules or plans.
The difference between must and have to concerns only affirmative sentences and not negations or questions because must does not appear in these sentences. Instead we use have to:
And at last: we can use have to in all tenses and forms. Must is related only with the present.
The negation of the verb must sounds mustn't and it means that we categorically deny doing something:
The form needn't has similar meaning to not have to:
Talking about the past we use needn't + have + past participle:
It's good to remember that should does not express orders such as e.g. must:
Talking about an obligation in the past we use should + have + past participle:
Ought to and should have exactly the same meaning. Should is more common than ought to. Ought to is more formal than should.
Now let's have a look at some sentences with ought to:
As we know we use the verb will to express the future (Future Simple). We shouldn't forget it's also a modal verb.