1. I can swim well.

Question

2. I could swim well.
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What is the difference between the two sentences?
Is 'could' politer than 'can' in meaning?

Answer 1

could is the past tense of can

I can swim well.
Yesterday I could swim well.

Then there are the conditional and perfect uses, but maybe you're not after those.

Answer 2

Steve is correct.

In addition, this webpage should have additional information you can use:
http://grammar.ccc.commnet.edu/grammar/auxiliary.htm#can