The worst thing he can/could call you is a coward.

1. The worst thing he can call you is a coward.
2. The worst thing he could call you is a coward.

Which one is correct or better?

They're both grammatically correct and possible. It depends on context, as always.

emsr2d2 said:

They're both grammatically correct and possible. It depends on context, as always.

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What’s the difference?

MichaelLu2000 said:

What’s the difference?

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Context! As I already said. Why don't you give us a situation in which you might want to use the sentence and then we can see whether we would choose "can" or "could"?