could 表建议
it is a nice day. we could go for a walk. 今天天气不错，我们可以出去散步
shall用于第一人称疑问句，表征求对方建议
shall i open the window? = would you like me to open the window?
may(just) as well/might(just) as well
“倒不如”“不妨”
should/ought to 表建议、忠告
could比should表建议时的语气要弱得多，只是为听话者提供一种参考意见，听话人可以有多种选择。而should则是表示听话人最好应该采用这一种选择。
A:i need to get to the airport.
B1:you should take the airport bus,it's cheaper than a taxi.
B2:well,you could take the airport bus.or you could take a taxi.maybe clint could take you. he has a car.
shoud还可表义务
i should have done a better job.
had better 表示忠告，暗含“如果不这么做，就会有不良的后果”
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