看完这段在看看下面词典里的解释和例句你应该就懂了.
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.
could
could
AHD:[k‹d]
D.J.[k&d]
K.K.[k&d]
v.aux.（助动词）
can1的过去式
Used to indicate ability, possibility, or permission in the past:
用来表示过去的能力、可能性或允许：
I could run faster then. It could be no better at that time. Only men could go to the club in those days.
那时我能跑得更快.那时好得不能再好了.在当时,只有男人可以去俱乐部
Used with hypothetical or conditional force:
用来表推测或条件：
If we could help, we would.
如果我们能帮上忙,我们会帮的
Used to indicate tentativeness or politeness:
用于表示试探性或礼貌：
I could be wrong. Could you come over here?
我也许错了.你能到这边来吗?
should
should
AHD:[sh‹d]
D.J.[.&d]
K.K.[.&d]
v.aux.（助动词）
shall的过去式
Used to express obligation or duty:
应该：用来表示义务或职责：
You should send her a note.
你应该给她留一个条
Used to express probability or expectation:
用于表示可能性或期望：
They should arrive at noon.
她应该在中午到达
Used to express conditionality or contingency:
用于表示可能性或可能发生的事件：
If she should fall, then so would I.
如果她失败了,那我也会失败
Used to moderate the directness or bluntness of a statement:
用于使一个直接或直率的陈述变得婉转：
I should think he would like to go.
我倒是认为他愿意去 .
