I AM quite confused about using “could you”, “can you”, “would you” and “will you”.

One day, I asked my school teacher about it and she told me there was no difference at all. She said if we used “would you” and “could you” to ask questions, that would show better manners.

After what she said, I wondered whether she was right.

What about sentences like “I could be a doctor one day” and “I can be a doctor one day”? The same applies to “would” and “will”. Can you explain these to me?
If you're asking a favor, that is, you want someone to do something for you, use Could you? or Would you?

Could/Would you please pass me the salt?
Could/Would you do me a favor?
Could/Would you please help me lift this?

You can use Can or Will in all three above, but Could and Would, especially when you add please, are a little more polite.

If you're asking for information, use Can you? or Will you?
Can you? asks about an ability. Will you? asks about the future.

Can you speak Spanish?
Will you be at the party tonight?

Would indicates that you are willing to do something. Will is about the same but a little stronger. Could means you have the ablity. Can means "to be able" so asking if you can do something is asking if you have the ability. For example, Can you speak French? means do you know how to speak French.

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 CalifJim's reply was promoted to an answer.
if you still confuesd from what is the right one i'm gonna give you the answer , but you should be carefull and pay attention , firstly what are going to talk about " the use of " would" ? we use that kind of latter models , for example would you close the door ? this expression when we wann ask somebody kindly for have favour to be done for us , and about can' we use it if only wanna ask the person if he or she'll be able to do the work, namely the abiltily ,and not forgetting could , it has both meaning, the first for expressing the past ablitiy and the second for asking for help kindly , concerning the last one , will' that kind models is specifically used to mean a promise or maybe for anticipating somethin' that probablelly happens