Hi Hyomin, "Do you speak English?" is a more polite way of asking. "Can you speak English?" is asking whether someone is able to speak English, and it is technically correct. However, it can be seen as an insulting way of asking someone, so most people say "Do you speak English?".

To say "he can speak English well, but doesn't understand it at all" does not make any sense, since "speaking English" implies an facility with the language. However you could say "he can speak in English well, but he can't understand it" is semantically acceptable, since speaking is different than understanding. In everyday speech, they are often used with the same meaning. However, #1 really refers to habit/custom/everyday routine, while #2 refers to ability. eg can speak Italian, but I do not speak Italian because I don't know any Italians or ,I can speak Italian, and I do speak Italian because I work in Italy.

Do you and can you are interchangable really. I'd normally say "Do you speak....". However both sound ok.

Hello

Both mean the same thing. "Do you..." is the most natural. "Can" is only rude if the tone is hostile. As with so many things, it's based on the context.