How to write a warning letter that employees always do something wrong at work? urgent

That depends on why he did something wrong, intentionally or unintentionally. If he did it on purpose, there is no need to mention it. He didn't regard you as a leader at all, just gave him a scare and gave him a serious warning. If it is an unintentional mistake, please be polite. Confirm his usual achievements and performance first, and then remind him not to make the same mistake again in the future. I'll ask you directly if you have any questions.