All the doctors have to do is heal the injured and rescue the dying.
医生要做的就是救死扶伤。
The only thing we could do is watch him going away.
我们唯一能做的就是眼看着他离开。
（4）在介词 but，except 后
He did nothing all day except smoke.他整天就知道吸烟。
The boy can do everything but read.除了读书这个男孩什么都做。
（5）在 cannot but，cannot choose but 和 cannot help but 之后
According to the doctor，the husband cannot but wait outside.
根据医生嘱咐，这位丈夫不得不在外面等候。
The soldiers could not choose but surrender.士兵们不得不投降。
（6）在 had better，would rather，would sooner，had best，rather than，other than 等短语之后