could和should的区别：
1. should可以用于第一人称或第三人称的疑问句中，表示征求意见。
例如： My brother likes to listen to CDs. He often plays his CDs too loud. What should I do?
2. should 表示义务，可以用于提供建议，表示“应该”, 后面跟动词原形。
例如： Maria argued with her mother last night. She is very upset now. What should she do? I think she should say sorry to her mother.
3. should也用于表示某事发生的可能性，用于表示推测的词，意为“可能”。
例如： They should arrive soon.
4. could不仅仅是充当can的过去式，而且是一个单独的情态动词，表示“能；可以”，用来提供建议，后面跟动词原形，它和should在意义上不同，前者是“可以”，后者是“应该”，表示不同的态度，但两个都可以用来提供建议。
例如： You could borrow a bike to that park. I think you should have a talk with your friend.
