This is FAR too big and too general a question to answer here! Do you really think that we can explain this major area of grammar in just a few sentences? You need to look for 'modal verbs' in a grammar book or on a language website and spend some time studying what it says. Then, if you have a specific problem about a specific sentence or two, you can come back and ask us a more precise question. Good luck. Here's one thing you can take home from this conversation, though: Your question should be 'What's the difference between....?' Different = adjective Difference = noun

"Can" is one of the most commonly used modal verbs in English. It can be used to express ability or opportunity, to request or offer permission, and to show possibility or impossibility. "Could" is used to express possibility or past ability as well as to make suggestions and requests. "Could" is also commonly used in conditional sentences as the conditional form of "can." "Will" is used with promises or voluntary actions that take place in the future. "Will" can also be used to make predictions about the future. "Shall" is used to indicate future action. It is most commonly used in sentences with "I" or "we," and is often found in suggestions, such as "Shall we go?" "Shall" is also frequently used in promises or voluntary actions. In formal English, the use of "shall" to describe future events often expresses inevitability or predestination. "Shall" is much more commonly heard in British English than in American English; Americans prefer to use other forms, although they do sometimes use "shall" in suggestions or formalized language "Would" is most commonly used to create conditional verb forms. It also serves as the past form of the modal verb "will." Additionally, "would" can indicate repetition in the past "Should" is most commonly used to make recommendations or give advice. It can also be used to express obligation as well as expectation. This website will give more detailed info. http://www.englishpage.com/modals/modalintro.html Let me know if you have more problems with these words.

Could, should and would are past forms of these verbs, e.g. I can swim now, but I couldn't swim when I was a child.