When should I use can? When should I use could?
What is right under what context?
Since your name seems Indian, I'll also mention a common Indian-English idiosyncrasy that may clear up matters for you. There is a tendency in Indian speech to use "could" for "can", and "would" for "will". This is wrong (or, to avoid being prescriptive, certainly at variance with other varieties of English, and non-standard even in India). Properly, "could" (subjunctive) is used to express possibility, things that may or may not happen, may or may not be done, etc. (to ask polite questions, for instance) — or in the past tense. If you're using it in the same sense as "can" (for a straightforward expression of ability), you're probably using it incorrectly.
To keep it simple, I answer you without complex grammatical terminology. There are five possible situations of using can.
Could is used in two more ways where can isn't normal.
1. To make suggestions
2. To express, forcefully, what someone must do
I hope that this answer could help you a lot! :)
"Could" is the subjunctive form of "can." That means you use it to express possibilities and the like. "I could go to the movies, but I might just stay home."
When "could" is used as the past-tense of "can," you're talking about something you used to be able to do, but can't anymore, so whatever action you're speaking of is hypothetical. "I could have gone to the movies, but I decided to stay home," or "I could mow the lawn before my back injury." (Really that second sentence should say "I could have mown...," but a lot of people don't bother.)
The subjunctive is also used when talking about emotions, wishes, judgements, and such like. Honestly, Wikipedia is probably better at explaining the subjunctive than I.
The difference is one of mood and tense. kitukwfyer hits all the right notes here.
The subjunctive also helps differentiate the forms in questions. Compare:
Could lends politeness to a question in a request of someone. I’d more likely say, “could you help me”, than “can you help me?”.
Addition to kitukwfyer's answer. Could is consider more polite than Can, for example, when you talk to your friends you can say, "Can you give me that pen?", however when you talk to your teacher you should say "Could you give me that pen?"
'could' is not the past tense of 'can'. In modern English modal verbs are tenseless. That's why they can all operate in all time sequences.
protected by RegDwigнt♦ May 19 '11 at 8:20
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