problem with grammar

can
動詞的現在式多用於表達 常用/常見/肯定的/定律/自然現象等
eg. He can play guitar. / I can walk. / Smoke can kill. / Bird can fly...

用於發問則表現平常語氣,
eg. Can you play quitar? / Can you walk?. / Can we not to talk about ghosts?

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而could則用於表達 不肯定/不常見/不可能等
eg. Sodium plus water could explode. / A dolphin could swim faster than a ferry.
I could have killed that man if the gun was not empty.

用於發問則表示 不市定/不相信/有禮貌
eg. Could it be cheaper? / Could a man run faster than a bus? / Could you pass me the salt?

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雖然could係比較有禮貌, 但並不表示can冇禮貌

'Can' is present tense.
'Could' is past tense.
Examples
I can swim.
I could play the piano when I were six.