Is there any differences between "can't you swim?" and "can you not swim?",thank you.
"Can't you swim?" and "can you not swim?" are both used to ask someone if they can swim or not. The contracted form is used more often, so the uncontracted form carries a little more emphasis, like when you can't believe that someone can't swim.
In addition, "can you not swim?" can be used to ask someone to stop swimming. (See also can you not in Urban Dictionary.) Here's a similar example, where the speaker is asking the other person to stop talking:
"Please. Just steer the boat and call the drills. Can you not talk so much?" —The River Killings