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If you say that someone could do something, you mean that they have the ability to do it, but they don’t in fact do it. The use of can to ask or grant permission has been common since the 19th century and is well established, although some commentators feel may is more appropriate in formal contexts. May is relatively rare in negative constructions (mayn’t is not common); cannot and can’t are usual in such contexts. You use could or a past form of be able to to talk about ability in the past. Can, could, and be able to are all used to talk about a person’s ability to do something. You use can or be able to to talk about ability in the present.

Be able to is more formal than can. Don’t use ‘could not have’ to say that there is a possibility that something was not true. Instead you use might not have or may not have. You also use could have to say that there was a possibility of something being true in the past, although it was not in fact true. You use could have to say that there is a possibility that something was true in the past. Don’t use ‘could not’ to say that there is a possibility that something is not true.

• You use can or be able to to talk about ability in the present.
• Can, could, and be able to are all used to talk about a person’s ability to do something.
• You also use could have to say that there was a possibility of something being true in the past, although it was not in fact true.

Instead you use might not or may not. You use could to say that there is a possibility that something is or will be true. Could and can are used to talk about possibility in the present or future.

• You use a future form of be able to to talk about ability in the future.
• Don’t use ‘could not’ to say that there is a possibility that something is not true.
• Could and can are used to talk about possibility in the present or future.
• These uses are dealt with separately in this entry.

These words are used to talk about ability, awareness, and possibility. They are also used to say that someone has permission to do something. These uses are dealt with separately in this entry. You use a future form of be able to to talk about ability in the future. Could is also used to talk about ability in the present, but it has a special meaning.

Can and could are used with verbs such as see, hear, and smell to say that someone is or was aware of something through one of their senses. If you say that someone was able can you smoke magic mushrooms to do something, you usually mean that they had the ability to do it and they did it.