The key answer of exercise 32

ABILITY IN THE PAST

• could + base form; Could + subject + base form?
• couldn't (could not) + base form = wasn't / weren't + able to + base form
• was / were + able to + base form; Was / Were + subject + able to + base form?
• managed to + base form; didn't manage to + base form; Did + subject + manage to + base form?
• succeeded in + -ing form; didn't succeed in + -ing form; Did + subject + succeed in + -ing form?

USE 

• General abilities in the past - Example: She could swim when she was five.
• In their negative forms, could and was / were able to can be used interchangeably
• A specific ability or an action that has been successfully concluded - Example: I was able to fix the car yesterday.
• A specific achievement that was difficult, unexpected or uncertain - Example: She succeeded in climbing the mountain.

MODAL VERBS OF DEDUCTION (past)

USE To make logical deductions about past events and situations

100% CERTAINTY → we are certain about something in the past 

• subject + must + have + past participle - Example: He must have forgotten about the meeting.

50% UNCERTAINTY AND POSSIBILITY → we are unsure or open to various outcomes in the present or future

• subject + may / might / could + have + past participle - Example: They are still at the platform. They may have missed the 6.30 train.
• subject + may / might + … have + past participle - Example: She might have left her keys at home.

We do NOT use could not or couldn't.

0% IMPOSSIBILITY → we consider something to be impossible based on existing knowledge or conditions

• subject + can't / couldn't + have + past participle - Example: Tom can't have been at school yesterday. He was ill.