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1. The past tense of can: used to indicate past ability, possibility, or permission; the negative form is: couldn’t or could not.
I could run faster then.
At that time, I could run faster.
She said that she couldn’t come. She said she couldn’t come. I couldn’t hear what they were saying. I couldn’t hear what they were saying. Sorry, I couldn’t get any more. Sorry, I couldn’t get any more. It could be no better at that time. At that time, it couldn’t have been better. Only men could go to the club in those days. Back then, only men could go to the club. Don’t worry─they could have just forgotten to call. Don’t worry, they might have just forgotten to call. 2. Politely asking questions or stating opinions, at this time, could and can have no difference in tense. You can use can, but using could is more polite, humble, and courteous.Could you please do it for me? Could you help me with this? Could you babysit for us on Friday? Could you watch the kids for us on Friday? I could be wrong. Could you come over here? I might be wrong. Could you come over here? It could be about 4:30 p.m. It might be around 4:30 p.m. 3. Making suggestionsWe could write a letter to the director. We might as well write a letter to the director. You could always try his home number. You can always try calling his home number. 4. Could is used in the main clause of conditional and hypothetical sentencesIf he worked hard last week, he could finish the work. —->This is a real conditional sentence, where could in the main clause indicates the future, making the tone more polite.If he had worked hard, he could have finished the work. —->This is a hypothetical sentence In reality, he didn’t work hard, so he failed to complete the work. 5. could have + past participle: indicates a judgment about past things, can be translated as ‘could have’ or ‘might have’, implying an action that was not realized at that time.They could have let me know they were going to be late! They should have told me earlier if they were going to be late! Anyone could have drugged that wine. Anyone could have put drugs in that wine. You could have been killed! You could have been killed! He could have taken a spare key. He should have taken a spare key.
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