if you contrast one thing with another, you point out or consider the differences between those things.
简单说，对比就是比较两个事物的不同之处，以达到强调的作用。比如官方指南第四篇写作题 Let There Be Dark 中有这样一句话：
And too little darkness, meaning too much artificial light at night, spells trouble for all.
1)澄清观点to elucidate ideas and clear their meanings2)加强观点
opposite and contrasting ideas shock the audience, heighten drama and are memorable for readers.
3)平衡观点contrasting ideas produce balanced structures.
What the author does?
The author employs a distinct contrast…
The author contrasts the fact with another…In contrast to…
Why he or she does it?
…to highlight a paradox…
This tool is utilized to establish an emphasis on…What effect this is likely to have on readers?
…contribute to the sense that…
…clarify the concepts for the readers.
如何使用3W原则来分析一篇议论文中的contrast呢，我们一起看个例子吧~We were taking the black young men who had been crippled by our society and sending them eight thousand miles away to guarantee liberties in Southeast Asia which they had not found in southwest Georgia and East Harlem. And so we have been repeatedly faced with the cruel irony of watching Negro and white boys on TV screens as they kill and die together for a nation that has been unable to seat them together in the same schools. And so we watch them in brutal solidarity burning the huts of a poor village, but we realize that they would hardly live on the same block in Chicago. ( 原文选自：Beyond Vietnam — A Time to Break Silence.)分析King sees young black men, denied their rights at home, sent 8,000 miles “to guarantee liberties in Southeast Asia”; Negro and white boys, who cannot study at the same schools in the US,“kill and die together” in Vietnam; boys who cannot live in the same Chicago neighborhood burn huts abroad in “brutal solidarity”. Dichotomies of contrasting images allow King to show how the war highlights state hypocrisy and the moral inconsistencies of segregation.