一定：certain.
例句：
我一定会时刻睁大眼睛提防他的行为。
I shall certainly keep a beady eye on his behaviour.
这项工作必须在一定期限内完成。
The work must be completed within a certain time limit.
我觉得我们一定是在什么地方拐错了路。
I think we must have taken a wrong turning somewhere.
一定要找个宽敞地儿坐得舒服一点。
Make sure you have plenty of room to sit comfortably.
一定要确保所有的门都锁好了再走。
See that all the doors are locked before you leave.
luanqiqing
