Can vs Could: Ability, Permission and Politeness

Can and could look like a simple present-and-past pair, and sometimes they are. But could does far more than report the past: it softens requests and it flags possibility that is less than certain. Learners who treat could only as "the past of can" miss most of its everyday work. The clear way to hold them apart is by job, not by tense.

Ability: here the pair really is present and past

For talking about what someone is able to do, can is present and could is past.

• I can speak three languages.
• When I was young, I could run for miles.
• She can drive, but she could not drive last year.

This is the one use where could is genuinely the past of can. If you mean an ability that existed before, could is correct.

Permission: both work, could is softer

Both can and could ask for or give permission, but they differ in tone.

• Can I leave early today? (normal, direct)
• Could I leave early today? (more polite, a little more formal)

Neither is wrong. Can suits friends and casual settings; could suits a boss, a stranger or any moment where you want to sound respectful.

Requests: could is the polite default

When you ask someone else to do something, could makes the request gentler.

• Can you help me? (fine, direct)
• Could you help me? (softer, more courteous)

Here could carries no past meaning at all. It simply adds politeness, which is why it is the safer choice when you are not sure how formal to be.

Possibility: can states, could hedges

Both can express possibility, but with different confidence.

• Winters here can be very cold. (a general, factual possibility)
• It could snow tonight. (a specific, less certain possibility)

Can tends to state what is generally possible; could points to a particular outcome that may or may not happen. If you are hedging about one situation, could fits.

Quick guide

Ask what you mean. Past ability? Could is the past of can. Asking permission or making a request, and want to sound polite? Could. Talking about a general possibility? Can. A specific maybe? Could. One short exchange shows the range: "Could you check if it can be fixed? It could take a while."

Modal verbs like these are subtle, and using them naturally is a clear sign of control over English. The related modal pair shall vs will works in a similar way. A free English level test will tell you whether distinctions like this already come automatically to you.