Here's some examples
"I could help you with your work" gives a tentative sense of ability to help. There might be an implicit conditional by using 'could' in this sentence instead of 'can'. Using 'would' in this case would give the sentence a sense that there might be something preventing you from helping. I think it would sound funny to use 'would' and not continue the sentence.
"John is not in the office today, he could be sick."
In this case, it would not make sense to use 'would' because the sentence expresses a possibility.
"In the 1960s, people thought we would all be driving hovercars by the year 2000."
In this case, using ‘would’ implies that people were certain. Using 'could' in this case would indicate they thought there was only a chance people would all be driving hovercars.
"I would like a glass of water, please."
In this case, it would not make sense to substitute 'could'.
I could leave.
In this case, the speaker is making a suggestion and it wouldn’t make sense to use 'would'.