Question 1

What is the difference between could and was able to?

Accepted Answer

For general past ability — something you could do at any time — both "could" and "was able to" work: "I could swim when I was young" = "I was able to swim when I was young." However, for a specific achievement — one particular occasion where someone succeeded — use "was able to," not "could": "After three hours, the firefighters were able to put out the fire" (they succeeded on that occasion). You cannot say "the firefighters could put out the fire" for a specific event. The exception is with verbs of perception (see, hear, feel): "I could see the mountains from my window" is correct for a specific occasion because these are not active achievements.

Question 2

When do I use could for suggestions?

Accepted Answer

Use "could" to make gentle suggestions — offering possibilities without pressure: "We could try that new restaurant" or "You could ask your teacher for help." This is softer than "should" (which gives advice) and less direct than "why don't you" (which is more pushy). The person is free to accept or reject the idea. You can also use "could" to present options: "We could stay here, or we could go home." This is very common in spoken English when making plans with friends or colleagues. The negative "couldn't" is not normally used for suggestions — instead, say "we could always" for emphasis: "We could always cancel if the weather is bad."

Question 3

How do I use couldn't for deduction?

Accepted Answer

When you want to say that something is impossible based on evidence or logic, use "couldn't" or "can't": "That couldn't be Sarah — she is in Spain this week" (it is impossible, I know she is away). This is the opposite of "must be" (strong positive deduction): "He must be tired" (I am almost sure) vs "He can't be tired" (it is impossible — he slept for ten hours). For the past, use "couldn't have" + past participle: "She couldn't have stolen it — she was with me all evening." This pattern is frequently tested in Cambridge B2 First and IELTS exams, so it is important to practise.

Could Exercises

Quick Rule

Continue Practising

Can / Can't

Must / Have to

Should

-ed vs -ing Adjectives

Conditionals

Present Perfect