be sure to do；
be sure of sth.doing sth；
be sure that +从句
The worst of it is that I can\'t even be sure if they received my letter.
最糟糕的是，我甚至不能确定他们是否收到了我的信。
It is impossible to be sure about the value of land.
土地的价值是无法确定的。
Be sure you get your daily quota of calcium.
要确保每天的钙摄入量。
