"Can" expresses ability or possibility in the PRESENT; "could" expresses past ability, theoretical possibility, or a more polite request.
CAN: PRESENT modal. (1) PRESENT ABILITY: "She can speak four languages" = she has this ability now. (2) PRESENT or GENERAL POSSIBILITY: "It can be very cold in January." (3) Informal PERMISSION: "Can I use your pen?" (4) Informal REQUEST: "Can you help me?" Structure: CAN + bare infinitive (no "to"): can do, can go, can help.
She can analyze large datasets efficiently — that skill makes her one of the most valuable members of the analytics team.
Can you send me the presentation slides before the end of the day? I need to review them before the client meeting.
The project can be completed in six weeks if the team works at full capacity throughout the sprint.
COULD: four main uses. (1) Past ABILITY or POSSIBILITY: "She could run a marathon at 25" = she had this ability then (perhaps no longer). (2) THEORETICAL POSSIBILITY (more tentative than can): "It could rain tomorrow" = possible but less certain. (3) More POLITE REQUEST than can: "Could you help me, please?" = more formal and respectful. (4) More FORMAL PERMISSION: "Could I use your pen?" = more polite than "can I". (5) CONDITIONAL form of CAN: "If I had more time, I could do it."
When he was younger, he could work fifteen-hour days without feeling exhausted — those days are well behind him now.
Could you review the draft proposal before tomorrow's meeting? Your perspective on the financial section would be especially valuable.
The delay could affect our Q3 targets if we do not find a way to accelerate the remaining phases of the project.
Common mistake
Most common errors: (1) Using CAN for requests in FORMAL contexts. "Can you please review my proposal?" in a professional email sounds informal. In professional English or with superiors, use COULD: "Could you please review my proposal?" (2) Confusing COULD (past ability) with MANAGED TO (specific past achievement). "She could finish the project on time" = ambiguous: it may mean she had the capacity or that she did it. "She MANAGED TO finish the project on time" = she succeeded (emphasis on specific achievement). (3) "I can could do it" = INCORRECT. Can and could are different forms of the same modal — never used together.
Trick to remember it
The FORMALITY DIAL and the TIME TRICK: TIME: CAN = present/future. COULD = past or more tentative/polite. "She can run fast" (now). "She could run fast" (before, or: possibly she can). FORMALITY: CAN = informal. COULD = formal/polite. "Can I ask a question?" = normal. "Could I ask a question?" = more polite. For REQUESTS: in work emails, always COULD. With friends, CAN is natural. Scale: Would you mind...? (most formal) > Could you...? > Can you...? > Do you want to...? (most informal).
Quick rule
Can = PRESENT ability/possibility, informal request. Could = PAST ability, THEORETICAL possibility, FORMAL/POLITE request, or conditional. In formal professional contexts, use could for requests.
Did you know?
The CAN/COULD distinction in requests is a fascinating sociolinguistic marker: pragmatics studies show that the type of modal used in a request instantly signals the degree of familiarity, hierarchy, and formality between speakers. In British office English, a superior saying "Can you do this?" to a subordinate can sound almost like a command; the same superior using "Could you do this?" signals acknowledgment of the imposition of the request. This subtlety does not exist in the same way in Spanish, where "puede" and "podría" are more interchangeable in formal contexts.
Exercise 1 · Pick the right one
When he was a professional swimmer, he _____ complete a 1,500-meter race in under eighteen minutes.
Exercise 2 · Pick the right one
"_____ you take a look at my presentation draft before the meeting? I want to make sure the data visualization is clear."
Exercise 3 · Form the word
Can = PRESENT ability/possibility, informal request. Could = PAST ability, THEORETICAL possibility, FORMAL/POLITE request, or conditional. In formal professional contexts, use could for requests.
Most common errors: (1) Using CAN for requests in FORMAL contexts. "Can you please review my proposal?" in a professional email sounds informal. In professional English or with superiors, use COULD: "Could you please review my proposal?" (2) Confusing COULD (past ability) with MANAGED TO (specific past achievement). "She could finish the project on time" = ambiguous: it may mean she had the capacity or that she did it. "She MANAGED TO finish the project on time" = she succeeded (emphasis on specific achievement). (3) "I can could do it" = INCORRECT. Can and could are different forms of the same modal — never used together.
The FORMALITY DIAL and the TIME TRICK: TIME: CAN = present/future. COULD = past or more tentative/polite. "She can run fast" (now). "She could run fast" (before, or: possibly she can). FORMALITY: CAN = informal. COULD = formal/polite. "Can I ask a question?" = normal. "Could I ask a question?" = more polite. For REQUESTS: in work emails, always COULD. With friends, CAN is natural. Scale: Would you mind...? (most formal) > Could you...? > Can you...? > Do you want to...? (most informal).
Bryan López
Founder of SpeakUP Academy