Key difference: ‘Could’ and ‘Would’ are the two words which are the past tense of the words ‘can’ and ‘will’. Basically, they are used to convey expressions; ‘could’ is generally used to express the possibilities in the future, while ‘would’ is used to express unlikely but possible situations.

The word ‘could’ is a verb form of ‘can’. It is basically used to express the possibility of any incidents. It is the simplest past tense of ‘can’, and is used to express the possibility, ability, polite requests, permission, suggestions, or advice.

For examples:

• John could go to the cinema tonight (express possibility).
• You could do it if you tried (express conditional possibility or ability).
• Could you open the book for me, please? (making polite requests).
• Could I borrow your car? (asking for permission).
• He could at least had called at the station (offering suggestions or advice)

The word ‘would’ is the past tense of the word ‘will’. Generally the word is used in the future tense, and denotes the future possibility of something that can happen. The word ‘would’ is basically used to express the future in the past tense, used in place of ‘will’ in order to make a statement or form a question less direct or blunt, to express repeated or habitual action in the past, to express an intention or inclination, to express a wish, to express an uncertainty, and conditional sentences to express choice or possibility.

For examples:

• She said, she would go tomorrow (express the future in past sentences).
• That would scarcely be fair. Would you be so nice? (in place of will, and in the statement or form a question less direct or blunt).
• We would visit Grandpa every morning up at the orphanage (express repeated or habitual action in the past).
• Nutritionists would have us all eat all fruits and vegetables (express an intention or inclination).
• Would John be here! (express a wish).
• It would appear that they are guilty (express an uncertainty).
• If the temperature were higher, the water would evaporate (in conditional sentences to express choice or possibility).
• I would have saved you some, but Jerry took it all (with a past participle to express unfulfilled intention or preference).

Comparison between Could and Would:

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