镇定下来

• calm down
• collect one's faculties
• sober

例句

1. 上讲台以前他先于自己镇定下来。
   He collected himself before getting up onto the platform.
2. 镇定下来吧，你这样紧张激动，完全是莫名其妙。
   Calm down, you're getting terribly wrought up over nothing.