waihaiss

1. 不用说，参加面试时应该穿上正式的衣服。
needless to say you should be dressed neatly when being interviewed for a job  
2. 不用说，空着肚子劳动是很难的。
needless to say ,it is difficult to do work for hungry
3. 不用说，启程旅行之前应该要先将车子加满油。
needless to say you should be fill up oil befor starting on our trip

waihaiss

why it is one ,not you .Needless to say, one should be dressed neatly when being interviewed for a job

thank you .!

because "one" stands for all persons,but"you"just stands for one person.  Thank you for your attention!