1. OBLIGATION MUST: IT’S NECESSARY/ IT’S COMPULSORY /IT’S URGENT.
EXAMPLE: You must be back at 10. Your mother is in hospital.
HAVE TO: EXTERNAL OBLIGATION (LAW , RULES).
EXAMPLES: You have to be back at 10.
I have to hand in this homework on Friday (EXTERNAL)
NEED TO: INFORMAL LANGUAGE
You need to study a lot = DON’T NEED .
BE CAREFUL!! THE PAST OF MUST AND HAVE TO: HAD TO.
She had to be back at 10, because her mother was in hospital.

2. LACK OF OBLIGATION: NEEDN’T: IT’S UNNECESSARY/ IT ISN’T NECESSARY
EXAMPLE: Jesús needn’t come on Sunday if he doesn’t want to.
DON’T HAVE TO: IT’S UNNECESSARY / IT ISN’T NECESSARY
EXAMPLE: Justo and Juan don’t have to take a taxi

3. LACK OF OBLIGATION IN THE PAST:
NEEDN’T + HAVE + PAST PARTICIPLE (-ED)= IT WASN’T NECESSARY TO DO SOMETHING BUT YOU DID IT
EXAMPLE: Rocío needn’t have come on Sunday but she came
DIDN’T HAVE TO + INFINITIVE =IT WASN’T NECESSARY TO DO SOMETHING AND YOU DIDN’T DO IT
Rafa and Jaime didn’t have to take a taxi. I took them home.

4. ADVICE: SHOULD /OUGHT TO
SHOULD: IT IS A GOOD IDEA TO DO SOMETHING
Pilar should go to the doctor, because she is ill.
OUGHT TO:
Pablo ought to drive more carefully.
SHOULDN’T/OUGHTN’T (NOT COMMON):IT’S NOT A GOOD IDEA
EXAMPLE: You shouldn’t smoke. It’s bad for your health.

5. ADVICE IN THE PAST SHOULD + HAVE + PAST PARTICIPLE:
IT WAS A GOOD IDEA TO DO SOMETHING BUT YOU DIDN’T DO IT.
Julia should have gone to the doctor, because she was ill.( But she didn’t)
SHOULDN’T + HAVE + PAST PARTIC.:
IT WASN’T A GOOD IDEA TO DO SOMETHING BUT YOU DID IT.
You shouldn’t have told your friend that (but you told him).

6. PROHIBITION:
MUSTN’T: IT’S FORBIDDEN/ IT’S DANGEROUS/ IT’S NOT ALLOWED.
EXAMPLE: Pepe, you mustn’t drive without a licence.
CAN’T/CANNOT: IT’S NOT SO STRONG. INFORMAL
EXAMPLE: Carolina, you can’t cross the road when the trafic light is red.

7. POSSIBILITY,PROBABILITY, SPECULATION
MAY /MIGHT/COULD: POSSIBILITY , BUT WE ARE NOT SURE.
EXAMPLES:
The sky is grey. Perhaps, it’s going to rain.
‘It may rain’= ‘It might rain’= ‘It could rain’, Teresa said.

8. SPECULATION IN THE PAST:
MAY + HAVE + PAST PARTICIPLE
That boy may have stolen the bag, he looks very suspicious. (Possibility or Speculation).
They might have studied at Oxford but I’m not sure. Possibility or Speculation
Jaime could have won the game, but he didn’t do it. Something was possible, but it didn’t happen.

9. DEDUCTION MUST: We are sure that something is true.
EXAMPLE: She must be Bárbara’s sister. They are so alike.
CAN’T: We are sure that something is untrue.
EXAMPLE: He has just eaten two pizzas, so he can’t be hungry

10. DEDUCTION IN THE PAST:
MUST + HAVE + PAST PARTICIPLE: I’M SURE THAT SOMETHING HAPPENED IN THE PAST
María must have travelled round the world; her passport has got many stamps. (Logical conclusion.)

11. DEDUCTION IN THE PAST:
CAN’T/COULDN’T + HAVE +PAST PARTICIPLE: I’M SURE THAT SOMETHING DIDN’T HAPPEN IN THE PAST
Alberto can’t/couldn’t have climbed Mount Everest. (Something was impossible in the past.)

12. ABILITY
CAN is used to indicate the possession of an ABILITY IN PRESENT
EXAMPLE: Juan Luis can speak German fluently / Isaac can’t speak German.
(= He’s able to speak German fluently)more formal.
ABILITY IN PAST: COULD
EXAMPLE: Isabel couldn’t swim when he was two years, but now, she can.

13. PERMISSION CAN /COULD/MAY:To ask permission and make polite requests.
EXAMPLE: Carlos asked his mother: Could/can you lend me ten euros , please?
Cristina asked her teacher: May I ask you a question?
CAN/CAN’T: To give permission in the present. (informal)
EXAMPLE: Álvaro : You can use my computer if you want.
Nacho: You can’t use your mobile phone in class.

14. PERMISSION IN THE PAST COULD /COULDN’T/ BE ALLOWED TO:
EXAMPLE: When Ignacio was 6 years old, he couldn’t/ wasn’t allowed to watch “The Simpsons” on TV.
EXAMPLE: When he grew up, he could watch “The Simpsons” on TV.