Both can and could belong to a category of verbs known as modal verbs — a special type of auxiliary verb that works alongside a main verb to express different shades of meaning. One of the most frequently asked questions among English learners is: what is the difference between can and could, and when should each one be used? The good news is that once you understand the core distinction, using them correctly becomes surprisingly straightforward.
What Is the Difference Between Can and Could?
While can and could share the same general meaning — expressing the ability or possibility to do something — the key difference lies in time and formality. Can is used to talk about present or general ability, while could refers to ability in the past. Additionally, could is considered more polite and formal than can when making requests or asking for permission.
How to Use Can and Could
1. Using Can to Express Ability
Use can to describe what someone is able to do in the present or in general.
Sentence structure: Subject + can + bare infinitive + …
Examples:
- They can sing very well. (They have the ability to sing well.)
- We can swim. (We are able to swim.)
When you want to say someone is unable to do something, use cannot or its contracted form can’t:
Sentence structure: Subject + cannot + bare infinitive + …
Examples:
- Emma can’t swim. (Emma is unable to swim.)
- I cannot drive. (I do not know how to drive.)
2. Using Could to Express Past Ability
Use could when talking about a skill or ability that someone had in the past — especially when implying that the ability may no longer exist to the same degree.
Sentence structure: Subject + could + bare infinitive + …
Examples:
- They could sing very well. (They used to sing well — perhaps not as much now.)
- I could speak English when I was ten. (I had that ability as a child.)
The use of could here subtly signals that the ability existed at a specific point in the past, adding a layer of meaning that can alone cannot convey.
3. Using Can and Could for Requests, Offers, and Permissions
Both can and could are widely used to make requests, ask for help, or seek permission. However, there is a notable difference in tone:
- Could sounds more polite and formal — ideal in professional settings or when speaking to strangers.
- Can sounds more casual and friendly — perfectly appropriate among close friends or family.
Adding please at the end of your sentence makes either form even more courteous.
Sentence structure: Can / Could + subject + bare infinitive + …?
Two people having a polite conversation, using formal and informal request structures
Examples:
- Could I use your toilet, please? (More formal/polite)
- Can I use your toilet, please? (More casual/informal)
- Could you turn off the light, please? (More formal/polite)
- Can you turn off the light, please? (More casual/informal)
Both versions are grammatically correct — the choice simply depends on the context and your relationship with the person you are speaking to.
Conclusion
The difference between can and could is not as complicated as it might first appear. To summarize: use can for present or general ability, could for past ability, and either one for making requests — keeping in mind that could is the more polite option. With a little practice, choosing between the two will feel completely natural in both spoken and written English.
References:
- Cambridge Grammar in Use – Raymond Murphy
- British Council: English Grammar — Modal Verbs
- GrammarLearn.com — Original source article on can and could