"Drive carefully"是表示"开车必须小心"的一种常用的英语短语。来源是因为开车时需要注意安全，所以要小心。例句：

"Please drive carefully on the road." (请在路上小心驾驶。)
"It's important to drive carefully to avoid accidents." (小心驾驶以避免事故是很重要的。)

在日常对话中，也可以简略地说"be careful when you drive"或者"watch out when you're driving"来表示"开车必须小心"的意思。例句：

"Be careful when you drive, there are a lot of pedestrians on the road." (开车的时候小心点，路上有很多行人。)
"Watch out when you're driving, the roads are spppery after the rain." (开车的时候小心点，雨后路面湿滑。)