|
| Meaning
| Use
| Present
| Past
|
|
| physical or mental ability or possibility
| +
-
?
| You can use computer for various purposes.
| We couldn’t use the floppy because of the viruses.
|
|
| permission
| +
| You can try a new application for this task.
| -
|
|
| suggestions
| +
| We can/could try another procedure instead.
| -
|
|
| offer
| ?
| Can I offer you this new product of IBM?
| -
|
|
| polite request
| ?
| Can/Could you help me with the database?
| -
|
|
| surprise or astonishment
| ?
| Can/Could you make such an inquiry?
| How can/could you have entered this data?
|
|
| doubt or uncertainty
| -
| -
| The processor can’t/couldn’t have come to a halt.
|
|
| certainty
| -
| They can’t/couldn’t be at our disposal.
| They can’t/couldn’t have been used for that purpose.
|
|
| prohibition
| -
| We can’t get access.
| They couldn’t use that devise.
|
|
| lost opportunity or complaint
| +
| -
| The floppy could at least have been checked for viruses before.
|
Notes:
1. Can has only two tense forms: can (present) and could (past), so for other tenses to be able to do smth. or to be capable of doing smth. are used. But if you mean that someone managed to do something in one particular situation (where can or could are possible), you have to use was/were able to do smth. or was/were capable of doing smth.
2. Could is not always the past of can, sometimes it has a present or future meaning. Perfect infinitive after can or could in such cases shows that they refer to the past.
3. Passive and progressive infinitives after can or could are also possible in some cases.
MAY – to be allowed (permitted) to do smth.
|
| Meaning
| Use
| Present
| Past
|
|
| permission
| + /?
| You may use my ISP if you wish.
| -
|
|
| prohibition
| -
| You may not use my e-mail box.
| -
|
|
| possibility
| + / -
| It may/might be a faulty program.
| The modem may/might not have been switched on.
|
|
| suggestion
| +
| We might use other graphical software.
| -
|
|
| complaint
| +
| They might pay more attention to the instructions.
| He might have checked his e-mail at least sometimes.
|
Notes:
1. May has only two tense forms: may (present) and might (past), so for other tenses to be allowed (permitted) to do sth. is used.
2. Might is not always the past of may, sometimes it has a present or future meaning. Perfect infinitive after may or might in such cases shows that they refer to the past.
3. Passive and progressive infinitives after may or might are also possible in some cases.
MUST
|
| Meaning
| Use
| Present
| Past
|
|
| obligation (speaker’s authority)
| +
?
| They must provide us with all necessary data.
| -
|
|
| necessity (speaker’s thought)
| +
| He must work hard to learn this computer language.
| -
|
|
| prohibition
| -
| You mustn’t press any button.
| -
|
|
| certainty
| +
| He must be working in the Internet. I can’t call him.
| They must have already opened this file.
|
Notes:
1. Must has no other forms. To express past perfect infinitive can be used sometimes.
2. Had to is the past form of must which refers to a past obligation.
3. Need is used in its negative form to express the lack of necessity.
