1) I would have called you, but I didn't know your number.
2) I could have called you, but I didn't know your number.
Can I use 2 in place of 1? Do they mean same?
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could goes to a possibility. Would is a conditional. – Lambie Dec 13 '17 at 19:20
Edit after reading OP's comment to this answer.
The two sentences in the question are saying different things. In the first case would is certain, but in the second case could remains a possibility. So the second sentence reads like an excuse.
Referring to the examination question asked in comment, you might think the same applies, but it is a different situation because passing the examination is something outside of your control (only studying and attending are), so using would in the first sentence is wrong. I suggest the valid sentence is
I could have passed the exam if I had studied hard.
answered Dec 13 '17 at 18:10
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Thank you. Are not these two sentences equivalent? 1 ) I would have passed the exam if I had studied hard 2) I could have passed the exam if I had studied hard – user678 9 Dec 13 '17 at 18:20
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That is rather different. You have moved the goal posts, and they are not equivalent. – Weather Vane Dec 13 '17 at 18:26
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If would "means" certain and "could" means possibility then isn't 1) means that speaker is certain about the outcome if he had studied hard. So 1 is valid sentence but has different meaning than 2. – user678 9 Dec 14 '17 at 5:28
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Studying hard does not guarantee passing an exam. It can turn out that the subject was just too difficult. – Weather Vane Dec 14 '17 at 9:54
The first case is assuring someone that, had you had their number, you would have called.
The second makes no such assertion, but points out that because you didn't have their number, it was impossible for you to call.
This is because would expresses (potentially subjective) certainty (when used in this sort of situation - there is some variation due to the history of the word), while could expresses possibility.
