有一读者问： as, when和while皆表时间，但有何不同？
as常强调与主句同时，如：
(1)He trembled as he spoke.
有时与主句虽不同时，但相隔很近，如：
(2)As they came in, they set down their shovels.
when常表“当……之后”，如：
(3)Stop writing when the bell rings.
但也常表“同时”，如：
(4)It is cold when it snows.
(5)When I was going out, it began to rain.
while常表“在……期间”，如：
(6)Strike the iron while it is hot.
