Make certain (that) you lock the door when you leave. = Make certain to lock the door when you leave.你离开时一定要把门锁好。

We must make certain [=make sure] this doesn't happen again.我们必须确保这样的事情不再发生。

She made certain she thanked each of them for coming.她确保自己向每位来宾都表达了谢意。

Make certain of your flight times [=be sure that you definitely know your flight times] before you leave for the airport.去机场之前一定要弄清楚你的航班时间。