Unit 5: Possibility and Probability

Unit 5: Possibility and Probability
Perfect Modals
Possibility: May, Might & Could:
1.May, Might are used to for the present or the future
Ex: it may/might rain today.
Might is less likely than May. 
Ex: It might rain today. 
Negative form of Might:
Might not: Something negative is possible.
Note: Might not or mightn’t and may not (mayn’t is extremely rare) are used to say that it is possible something will not happen.
 	 Ex: 	She might/may not go to the party.
I mightn’t be home till late this evening.
2.Could: something is possible
Less than 50% certainty:
Ex: 	Where’s John? He could be at home.
Negative form: couldn’t or could not
Impossibility (negative only)
Ex: That couldn’t be true!
Note: We often use phrases like I think, I should think, I suppose, I guess, I reckon, etcbefore may, might and could to give our opinion that something is possible.
I think my dad might phone tonight.
I guess that shop may have what you want.
I reckon I could finish this work today.
 Probability: Must & Can’t:
1.Must: something is certainly true.
Ex: Mary isn’t in class. She must be sick. (Present only).
In negative sentences both must not and don’t have to (also don’t need to and need not) are used. Must not is used when the speaker expects or orders somebody not to do something. 
Ex: Passengers must not smoke until the signs have been switched off. 
The short form mustn’t is used especially in British English: 
Ex: You mustn’t leave the gate open.
2.Can’t: impossibility (negative only)
Ex: That can’t be true!
MUST BE, CAN’T BE is used to express deduction.
Ex: 
The story must be a joke. It can’t be true.
A: I need a rest.
B: But we haven’t been working long. You can’t be tired yet.
 Note: The negative of must be is can’t be, not mustn’t be.
PROGERESSIVE FORMS OF MODALS.
Present time:
MODAL + be + v-ing
Meaning: in progress right now.
Ex1: Let’s just knock on the door lightly. Tom may be sleeping. (right now)
Ex2: All of the lights in Ann’s room are turned off. She must be sleeping. (right now).
Past time:
MODAL + have been + v-ing
Meaning: in progress at a time in the past.
Ex1: Sue wasn’t at home last night when we went to visit her. She might have been studying at the library.
Ex2: Joe wasn’t at home last night. He has a lot of exams coming up soon, and he is also working on a term paper. He must have been studying at the library.
Perfect Modals:
Use
Examples
MAY/ MIGHT + HAVE + V3
To show we are guessing about something that happened in the past.
1. Why wasn’t Mary in class?
 She may have been sick.
2. Stella can’t find her phone; She could have left it at home.
3. The plane may/might been delayed.
4. She might/may not have had the letter yet.
COULD HAVE + V3
To show we had the ability to do something in the past but didn’t do it.
She could have gone to the zoo with her John but she decided not to.
MUST HAVE + V3
To show we are sure about something that happened in the past.
- That dress you bought is very good quality.
It must have been very expensive.
CAN’T / COULDN’T HAVE + V3
To show we are certain something did not happen in the past.
- John passed the examination. He didn’t study very much for it.
The exam can’t have been very difficult.
SHOULD/ OUGHT TO HAVE + V3
To say that somebody didn’t do something he / she was supposed to do.
1. Why were you at home yesterday? You should have been at work.
2. She shouldn’t have been listening to our conversation. It was private.
3. I wonder why Sue hasn’t written us. We should have heard/ ought to have heard from her last week.