When using a map,pay attention to the following things.为什么用u

1、When using a map,pay attention to the following things.
=When you are using/you use a map,pay attention to the following things.
=When 【you are using/you use】 a map,【you should】pay attention to the following things.
【you】和【use】是主动关系,因此用using.
知识链接：
在时间、条件、地点、方式、比较、让步等状语从句中,如果从句主语和主句主语相同,或主语是it时,从句的主语和谓语动词的一部分（多是be动词）经常省略.
这里,结合祈使句,设置状语从句的省略结构,两种省略同时并存,理解难度比较大.
在省略结构中,如果句子主语和从句动作是主动关系,就用v-ing形式；如果是被动关系,就用过去分词.再如：
While crossing the street,the boy was hit by a car.
If finished,the report should be handed in before this afternoon.
2、You must by No.6 bus
典型的汉式英语表达——by是介词,不能直接作谓语.可以改为：
You must 【go by 】No.6 bus.
or
You must 【go/ get there by】 No.6 bus.
祝你开心如意!