could ,should 有什麼不同?

could同should都係modal verbs...
佢地ge分別係:
could is used :
1. as the past tense or the conditional of can
eg: it could be a rainy day if the weather condition changed dramatically
2. as a more polite expression of can
eg: could i use your life jacket,too?

should is the past forms of shall and will respectively.
1.should can express advice
eg:the weather is getting cold,you should put on heavy clothes.
2.should can replace the word ''if'' in conditional sentences.
eg: if may controls her diet,she will be a slim girl.
should May control her diet,she will be a slim girl.
ps:after ''should'' use bare infinitive