1._____they wanted to go out,they could always count on me to stay with baby.
问题描述：
1._____they wanted to go out,they could always count on me to stay with baby.
B.If C.On condition that
if=On condition that 但为什么只能用If?
2.The train leaves at 6;00 pm.So I have to be at the station______5:40pm at the latest.A.until B.C.by
两者都是直到,但为什么只能用by?
答
1、if 是“如果”的意思,表示假设某种情况,这个句子的意思是“如果他们想要出去,可以叫我替他们看孩子”；
而 on condition that 在多数情况下是“只要”的意思,通常不放在句首.如：You can use the bicycle on condition that you return it tomorrow.只要你明天归还,自行车你可以拿去用.这道题用的是虚拟语气,明显是表示假设的,所以用if.
2、until 用于肯定句时,句中谓语动词常表示持续性动作,句意为“持续性动作到……（时）为止”She went on reviewing chemistry until（或till）after 12 o’clock at midnight.她持续复习化学直到半夜12点以后.
这道题中,i have to be at the station,我得到达车站,“到”的动作只是一时的,不是持续的,所以不用until.