ABILITY CAN and BE ABLE TO- present We use can or be able to say that someone has an ability Ex. James can/is able to play chess but he can’t/ isn’t able to ride a bike

ABILITY CAN and BE ABLE TO – past We use could or was able to to say that someone had an ability in the past Ex. He could/was able to read when he was three but he couldn’t/wasn’t able to catch a ball when he started school

ABILITY We don’t use could to talk about one occasion in the past but we can use couldn’t. Ex. She was able to (not could) come to the meeting but she couldn’t stay for lunch

ABILITY BE ABLE TO – other tenses Can is only used in the present tense and could is only used in the past. In all other tenses you use be able to.

ABILITY I will be able to give you a lift on my way to college. (future) They haven’t been able to contact Mary because of the storms. (present perfect) If you saved enough money, you would be able to visit me in New York. (conditional) They hope to be able to visit me next year. (infinitive)

OBLIGATION For obligation, we can often use must and have to: I must go now or I’ll miss the bus. or I have to go now or I’ll miss the bus. We use must to give orders or strong advice, including to ourselves You must tell me everything. (= I feel strongly about this) She must be home by midnight. (These are my instructions) You must come to the hotel one day. (= I strongly advise you to) I must go now. (I have decided to do this)

OBLIGATION When there is a rule or where the obligation does not come from the speaker have to is more usual: You have to pay to park your car here. (= this is a rule) I have to stay until the food is cooked. (= this is part of my job) We usually use have to for habits: I have to get up early to cook breakfast.

OBLIGATION We only use must in the present tense. In all other tenses, we use have to: I had to work every day. (past simple) I’ll have to work longer hours. (future) I avoided having to speak to him by crossing the street. (verb+ -ing) If I got the job, I’d have to buy a car. (conditional)

OBLIGATION MUSTN’T and DON’T HAVE TO Mustn’t means “ don’t do it” Don’t have to means “it’s not necessary to do it” I mustn't wear jeans at work. (= it is wrong to do this; it isn’t allowed) You don’t have to stay at school until you are 18. (= you are not obliged to but you can if you want)

DEDUCTION: CERTAINTY Talking about the present We use:  must when we know something is true  can’t/couldn’t when we are sure something is not true.

This man is looking at a letter with an Australian stamp which has just arrived for his son. DEDUCTION: CERTAINTY It must be from his girlfriend because she’s in Australia. (I am certain it is from her) It can’t be /couldn't be from his girlfriend because that’s not her handwriting. (I am certain it’s not from her)

DEDUCTION: POSSIBILITY Talking about the present We use:  might/may/could when we think something is possibly true  might not/ may not when we think something is possibly not true

DEDUCTION: POSSIBILITY The letter might/may/could be from his friend Tony, because he moved to Australia recently. (= I know that Tony lives there and it is possible not certain, that the letter is from him) It might not/may not be from anyone that we know. (= It is possible that it is not)

DEDUCTION: CERTAINTY &POSSIBILITY PRESENTTRUENOT TRUE CERTAINTY Must +infinitive without to Can’t/couldn’t +infinitive without to POSSIBILITY Might/may/could +infinitive without to Might not/may not +infinitive without to

DEDUCTION: POSSIBILITY Notice that could means the same as might and may, but couldn’t is different from might not and may not.

DEDUCTION CERTAINTY &POSSIBILITY All the verbs in the box above can also be followed by be +verb+ -ing: His girlfriend might be travelling home at this moment

DEDUCTION Talking about the future We also use might(not), may(not) and could when we are not certain about the future: James might go to see his girlfriend in Australia next month.

Steve is making a journey across Australia by car from Sidney to Perth

DEDUCTION: CERTAINTY Talking about the past We use:  must have when we are sure something is true: Steve must have arrived in Perth by now. (= I’m certain he has arrived)  can’t have/couldn’t have when we are sure something is not true: He can’t/couldn’t have got there yet because it will take at least two weeks (=I’m certain he hasn’t got there)

DEDUCTION: POSSIBILITY Talking about the past We use:  might have/may have/could have when we think something is possibly true: He might/may/could have stopped for a few days on the way. (=it is possible)  might not have/may not have when we think something is possibly not true: He might/may not have had time to do everything he wanted. (=it is possible he didn’t)

DEDUCTION: CERTAINTY &POSSIBILITY PASTTRUENOT TRUE CERTAINTY Must have +past participle Can’t have/ couldn’t have +past participle POSSIBILITY Might have/ may have/ could have +past participle Might not have/ may not have +past participle

DEDUCTION: POSSIBILITY Notice that could have means the same as might have and may have, but couldn’t have is different from might not have and may not have.