By Alena Lien, 


Click below to expand each section. 


Affirmative sentences 

[subject] + ["could"] + [base form of verb]

Please note: "Could" cannot be contracted with pronouns.

  • "I could stay up all night when I was younger. Now I'm sleepy by 9pm."

  • "You said we could play if we washed the dishes."

Negative sentences 

[subject] + ["could not"] + [verb]

Please note: "Could not" is contracted to "couldn't."

  • "It was so noisy in there, I couldn't hear myself think!"

  • "He couldn't lift them so he left them downstairs."

Full forms are used in formal contexts or for added emphasis.

  • "She was overcome with fear and could not speak."


["could"] + [subject] + [verb]

[question word] + ["could"] + [subject] + [verb]

  • "Could I borrow $20? I left my wallet at home."

  • "What could you have done to help?"

It is possible to make negative questions even though their use is quite specific. The structure for full forms and contracted forms are slightly different. However, contracted forms are preferred in general.

Contracted form:

["couldn't"] + [subject] + [verb]

[question word] + ["couldn't"] + [subject] + [verb]

  • "Couldn't you have waited for another 5 minutes?"

  • "Why couldn't you ask her?"

Full form:

["would"] + [subject] + ["not"] + [verb]

[question word] + ["would"] + [subject] + ["not"] + [verb]

  • "Could you not have waited for another 5 minutes?"

  • "Why could you not ask her?"


Although "could" is the past tense form of "can," it has other uses as well.

1.   To express what someone or something was able or allowed to do.

This is the past tense of "can."

  • "He hung up before I could say anything."

  • "When I lived in Malaysia, we could go out to eat at 2am in the morning."

2.   To refer to past statements in reported or indirect speech.

This is also the past tense of "can."

Past: "You can submit the assignment on Tuesday."

Present: "He said we could submit the assignment on Tuesday."

Past: "I can speak Mandarin fluently."

Present: "She said she could speak Mandarin fluently."

3.   To refer to past events with sense and mental verbs.

Also past tense of "can."

  • "We could only smell smoke when we entered the house."

  • "I went to greet him but he couldn't remember my name."

4.   To express disapproval or criticism, usually by saying what you think someone else should do. 

- With "could" or "could have + [past participle of verb]."

  • "You could try to look a little happier."

  • "He could have cleaned the house before I came back."

Express strong disapproval for what someone has done with "how could you?"

  • "You just left without telling me. How could you?"

  • "How could he think that it was the right thing to do?"

5.   To talk about possibilities or imagined situations.

Past possibilities - with "could have + [past participle of verb]."

  • "I could've been raised in America instead of Australia."

- This can also be used to express regret concerning things that did not happen.

  • "I could have had a successful career if I didn't get married so young."

  • "He could have been a lawyer or an engineer but he ended up becoming a lab tech instead."

Present or future possibilities - with "could."

  • "With the heavy rain, we could be stuck in traffic for a while."

  • "Your package could arrive anytime in the next few days."

Unreal or theoretical situations - with "if" and "could."

  • "If I had a girlfriend, we could be enjoying a stroll along the beach right now."

  • "We could buy a mansion if we won the lottery."

6.   To express strong possibility or certainty about the past.

- With "couldn't" or "couldn't have [past participle of verb]."

This is usually used when you have concluded from evidence that something is logically impossible.

Please note: "Could not" is actually the negative form of "must." The modal verb, "must," can be used to refer to possibility or certainty in the affirmative, however "must not" is usually used to talk about something that is forbidden in the present and future - not about strong possibility in the past.

  • "He must be the perpetrator."

  •       "No, he couldn't have been. He has a solid alibi."

However, it is possible to hear people say "must not have" instead in more informal contexts. The contracted form, "mustn't," sounds quite old-fashioned.

  • "He must not have turned off the lights last night."

7.   To be more polite and formal (compared to "can").

To ask for permission.

However, "can" is used to give or refuse permission.

  • "Could we go in to see her now?"

  •       "I'm afraid you can't just yet."

  • "Could I use your toilet?"

  •       "Yes, of course you can. It's just down the hallway on your left."

To ask someone to provide or do something.

  • "I'm sorry, I didn't catch that. Could you say that again?"

  • "If you could return the books by Friday, that'd be great."

8.   To make a suggestion.

  • "We could drive along the beach for a more scenic route."

  • "Could you ask for an extension for your assignment?"