modal verbs past

1. Batxillerat
Pepa Mut

2. 
 My boss called me early in the morning and said I
had to work all day at the store.
Obligation

3. 
 I could ride a bike when I was four, but I couldn’t
read or write.
Ability

4. 
 It is possible I didn’t hear the alarm.
I may /might/could not have heard the alarm.
Possibility

5. 
Absence of obligation
 Present  Past
-He didn’t have to buy
tomatoes. There were a lot
in the fridge.
-You didn’t need to buy
any books. You could use
the ones in the library.

6. 
 He must have forgotten our appointment. He was
supposed to come an hour ago. I’m sure he forgot.
Certainty (+)

7. 
 He can’t have failed the test. I’m sure. He has been
studying a lot. (we do not believe something)
 She can’t have been hungry. She had already eaten
two pizzas.
Certainty (-)

8. 
 Someone broke into my house. I didn’t lock the door.
I should have locked it.
 I don’t think Helen is getting the job. She was late for
the interview. She shouldn’t have been late.
Someone didn’t do the
right thing

9. 
 Ben can’t have committed the crime. The police say he
was somewhere else. (It is impossible, we are very sure)
near past
 Ben couldn’t have committed the crime. The police said
he was somewhere else. (It was impossible, we were
very sure)
it happened way back in the past
Can’t / Couldn’t have + part
(it is impossible)

10. 
Possibility, Certainty,
Mistakes