Can

"Can" is a modal verb used to express ability, possibility, permission, and requests. Here are the primary contexts in which "can" is used:

Ability:

Present Ability: "Can" is used to indicate that someone has the ability to do something in the present.

Example: "She can speak three languages."

General Ability: It can also refer to general abilities that are not tied to a specific time.

Example: "Birds can fly."

Possibility:

Present Possibility: "Can" is used to express that something is possible in the present.

Example: "It can get very hot in the summer."

Permission:

Asking for Permission: "Can" is commonly used to ask for permission in informal contexts.

Example: "Can I borrow your pen?"

Giving Permission: It is also used to give permission.

Example: "You can leave early today."

Requests:

Making Requests: "Can" is used to make informal requests.

Example: "Can you help me with this?"

Could

Could" is the past tense of "can," but it also has other uses beyond indicating past ability. Here are the main contexts in which "could" is used:

Past Ability:

General Past Ability: "Could" is used to talk about someone's ability to do something in the past.

Example: "When I was younger, I could run very fast."

Possibility:

Present/Future Possibility: "Could" is used to suggest that something is possible now or in the future, often implying a degree of uncertainty.

Example: "It could rain later."

Hypothetical Situations: It is used to talk about hypothetical or unreal situations.

Example: "If I had more time, I could travel more."

Permission:

Polite Requests: "Could" is used to make polite requests, often more formal than "can."

Example: "Could you please pass the salt?"

Suggestions:

Making Suggestions: "Could" is used to make suggestions or give advice.

Example: "You could try restarting your computer."