The use of 'of is quite extensive, can you be more specific? Is there a certain sentence you're confused about or do you mix it up with another preposition, such as 'at'? Could, refers to ability. It is the past tense form of 'can' and is also used in polite requests instead of can. Even though when you ask, "could you pass me the salt?" it literally means, 'do you have the ability' it is suggesting that you would like the person to use that ability for you. "Could you tell me the time?" is a common one we ask someone wearing a watch or holding a phone. We know they have the ability but wish for them to use that ability. Would is about willingness. We are acknowledging that they have not only the ability but also the choice. "Would you pick at 8 pm?" You know it's possible so are asking if they are willing. "I would go with you if I didn't have to work." You are willing but are choosing work as more important.