Question

walk down the road until

＿＿＿ he was walking down the road,he was singing a song.A.when B.until C.As D.That
为什么

Answer 1

表前后动作同时进行的有as,when,while等.

as表示一边.一边.前后的动作同时进行.
As he was walking down the road,he was singing a song. 边走边唱

when虽表示时间上同时,但when从句多服务于主句,表示主句发生的时间
when he was walking down the road,he was singing a song. 当沿着马路走的时候,他正在唱着歌.（感觉怪怪的吧,所以when不对）

while虽表示时间上同时,但往往有转折之意,前后句多对比
while he was walking down the road,he was singing a song.尽管.但是.
（这个句子就更怪了,嘿嘿）

建议：学语法,背诵难点的时候,最好背诵一个适合自己的例子