Assumption or deduction
Oh no! I put my bag down by the door and it´s gone! Someone must have stolen it!
Sorry I couldn´t get out to meet you last night but I had to babysit for some friends.
Look, there´s no one else on the tennis courts. We .... (bother) to book one in advance. (We did it but now we realise it was not necessary)
Look, there´s no one else on the tennis courts. We needn´t have bothered to book one in advance.
Surely you mustn´t be retiring this year? You look so young!
Surely you can´t be retiring this year? You look so young!
Someone might have come in quietly, picked it up and slipped out with it. It´s possible, you know.
One day, we might/may/will be able to cure many common illnesses.
Apparently there are still plenty of tickets left for this year´s festival, so we ..... (worry) about getting hold of some.
Apparently there are still plenty of tickets left for this year´s festival, so we don´t have to worry / needn´t worry about getting hold of some.
You don´t have to make up your mind about the job offer yet if you don´t want to.
You ... (left) it on the bus on the seat next to you when you got off. (It's a possibility)
You might have left it on the bus on the seat next to you when you got off.
It took a lot of work but in the end the mechanic was able to get the car going.
My work colleagues kindly took me out to dinner to celebrate my promotion, and I didn´t need to pay a penny.
How lovely to see you, Bill! And this must be your wife, Betty?
No, I .... (do), because I remember looking in it on the way here. (I'm sure I didn't do it)
No, I can´t have done, because I remember looking in it on the way here.
I managed to raise some money for charity last week. (SUCCEEDED)
I succeeded in raising some money for charity last week.
You needn´t apply / don´t need to apply for a visa to travel round this country, as far as I know.
After many failed attempts, they could rescue the trapped miners. (SPECIFIC situation in the past)
After many failed attempts, they were able to rescue the trapped miners.
you ought to
it´s advisable to do it
I .... (be) more careful with it, I know. (Criticism - past)
I should have been more careful with it, I know. Thank goodness there wasn´t much in it.
I love ..... stay in bed on Sunday mornings.
I love being able to stay in bed on Sunday mornings.
It was much too cold to go swimming when I was on holiday, so I .... (pack) my swimsuit after all.
It was much too cold to go swimming when I was on holiday, so I needn´t have packed my swimsuit after all.
I´ve just seen the weather forecast and it might not be warm enough for a barbecue after all.