Modal Auxiliary Verbs - Could  

The modal verb could is most often used as a past-tense version of can, indicating what someone or something was able to do in the past; it can also be used instead of can as a more polite way of making a request or asking for permission. Could is also used to express a slight or uncertain possibility, as well as to make a suggestion or offer.

When describing what a person or thing was physically, mentally, or functionally able to do in the past, we use could instead of can. For example:

We also use could instead of can when describing an ability that is desired or wished for. (This is known as the subjunctive mood, which is used for describing hypothetical or unreal situations.) For example:

Conditional sentences in the past tense are called second conditionals. Unlike the first conditional, we use the second conditional to talk about things that cannot or are unlikely to actually happen.

To create the second conditional, we usually use the past simple tense after the if clause, followed by would + a bare infinitive to describe what would be the expected (if unreal) result of the condition.

However, if we want to describe what we would be able to do under a certain condition, we can use could instead. For example:

We often use could in what’s known as a mixed conditional, which occurs when the tense in one part of a conditional sentence does not match the other half. This often occurs with could when a present-tense verb is being used in an if conditional clause to express a hypothetical scenario that is likely to or possibly could happen. For example:

When we ask someone for permission to do something, it is often considered more polite to use could instead of can. However, we can only make this substitution when asking for permission—when stating or granting permission, we can only use can (or, more politely, may).

For example:

Just as we use could instead of can to be more polite when asking for permission, it is also considered more polite to substitute could when making a general request. For example:

Note that we can also do this with the modal verb would:

Sometimes, we use could as a rhetorical device to politely introduce or emphasize an opinion or sentiment about something, in which case we invert could with the subject. For instance:

Note that we can accomplish the same thing by using the verbs let or allow instead, as in:

Like can, we can also use could to describe actions or outcomes that are possible or likely. Unlike using could to talk about an ability, this usage is not restricted to the past tense. For instance:

Similar to expressing a possible outcome, we can also use could to suggest a possible course of action. For instance:

We also use could to add emphasis to an angry or frustrated remark. For example:

In addition to using could to make a suggestion, we can also use it to make an offer to do something for someone. For example:

Could is sometimes used informally in sarcastic or rhetorical questions that highlight a behavior someone finds irritating, unacceptable, or inappropriate. It is often (but not always) used with be as a main verb. For example: