You can rest assured that these mistakes won't happen again. = Rest assured, these mistakes won't happen again.你就放心吧，这些错误不会再出现了。I think you're worrying unnecessarily. Just relax.我觉得你的担心是多余的，放心吧。Rest assured, he probably has rather more common sense than you realize.放心吧，他的常识很可能比你以为的要多。Don't worry, Seamus, I won't breathe a word to a living soul.放心吧，谢默斯，我不会在任何人面前走漏风声。