The best replacement for could is can.

The best alternatives to could are rated as following:
★★★★★  can
★★★★★  would
★★★★★  may
★★★  should
★★★  might
★★  can grammar
★  would grammar
★  was able to
★  couldn't
★  may grammar
★  would which is more polite
  would when to use
  able to
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  can exercises
  should esl
  would be able to
  we can

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Speaking about could, one probably shouldn't forget about wish.

Other search terms related to could and their ratings are:
★★★★★ wish
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