How to express certainty, uncertainty, probability, and possibility /impossibility:
  Certainty # Uncertainty:


 be sure to (do)...

 be bound to (do)...

 be certain to (do)...

 It is crystal clear that...

 I don't doubt that...

 There is no doubt about...

 It is impossible that...

yellowbl.gif  must

 I'm convinced that... 


 No one can deny that... 

 not be sure to (do)...

 not be bound to (do)... (informal)

 not be certain to (do)...

 be uncertain to (do)...

 it's possible

 it's impossible

 it might be

  it might not be

 it could happen

 I wouldn't like to say for certain.

 I'm not sure

 I doubt it

 I have my own doubts

 it's doubtful

 It's highly / very unlikely

 I don't think so.

 I think...  


 It's probable that ...

 Be likely to (do) ...

 may / might


 maybe perhaps 

 It's not probable that ...

 It's improbable that ...

 Be unlikely to (do) ...
 Examples: Reference to future:

 It's certain that she'll visit us next summer.

 She is bound to visit us next summer.

 I'm not sure he will come.

 I'm uncertain I will go on the picnic with them next Sunday.

 It's probable that he'll pass his exam. He's likely to pass his exam.

 It's probable that he won't arrive on time.

 He's unlikely to arrive on time.

 It's possible that it will rain later. It may rain later.

 It's doubtful that he'll go to the match.

 He might go to the match.
 Examples: Reference to present:

A: Where's John? He isn't in the living-room. 

B: He must be in the bathroom. He can't still be in bed.
A: Is Mr. Smith in his office?
B: It's half past nine, so he should be there.
A: Is the headmaster in the teacher's room?
B: I don't know. He may be there.
A: Are the children still awake?
B: They might be, but I doubt it. They went to bed an hour ago.
 Examples: Reference to past:

modal auxiliary+have+past participle:

 Look at this beautiful handwriting. A six-year-old can't have written this. His father or mother must have helped him.

 There's no birthday card from Linda. She has got such a terrible memory that she's bound to have forgotten it's birthday.

 A: Why isn't the secretary in her office?
     B: I don't know. She has probably gone to see the manager.
     A: I wonder why James is so late this evening?
     B: He wasn't feeling very well this morning, so he may have gone to the doctor's on his way home from work. 

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