Unit 2
Lesson 2
Student’s Book page 21
GRAMMAR
Modal verbs of obligation and prohibition
● English has several different ways of talking about obligation (something that is necessary), prohibition (something that we are obliged not to do), permission (something that we are permitted to do) and giving advice. Some of these use modal verbs such as must, should and can. Modal verbs do not change form in the present tense.
must/mustn’t
● We use must + the infinitive to talk about an obligation. We use mustn’t + the infinitive for a prohibition.
-I must do my homework. (The obligation comes from the speaker.)
-You mustn’t talk in the library. (You are obliged not to talk in the library.)
have to/don’t have to
● We can also use have to + the infinitive to talk about an external obligation.
-You have to take off your shoes before entering someone’s home. (The obligation comes from somebody else.)
● We use don’t/doesn’t + have to + the infinitive to say that something is not necessary or not obligatory.
-We don’t have to go to school on Fridays. (There is no obligation.)
can/can’t
● We use can + the infinitive to say that something is possible or permitted.
-You can have another slice of pizza if you want to.
● We use can’t + the infinitive to say that something is not possible or not permitted.
-You can’t come in yet – the floor is wet!
should/shouldn’t
● We use should + the infinitive to recommend an action or advise someone to do something.
-You should go up the Eiffel Tower if you visit Paris on holiday.
-You should drive carefully all the time.
● We use shouldn’t + the infinitive to advise someone against doing something.
-You shouldn’t point at people. It’s rude.
-Students shouldn’t leave their homework until late in the evening.