to undo the effects of such a decrease a candidate would need 30 more GMAT points than would otherwise have been necessary.
是要消除降分所带来的影响，申请者则要在GMAT中多考30分，而这本是不必要的。
第一步：断句子
[1] to undo the effect of such a decrease a candidate would needmore GMAT points [2]than would otherwise have beennecessary.
第二步：串成分
[1] [to undo the effect of such a decrease]（目的状语）a candidate（主）would need（谓）more GMAT points（宾）
目的状语：undo the effect of sth. 意为“消除……造成的影响”，状语部分译为“为了消除这种降分造成的影响”；
句子主干为虚拟语气，翻译为“申请者可能需要更高的GMAT成绩”；
译文：为了消除（这种）降分造成的影响，申请者可能需要提供更高的GMAT成绩
[2] 修饰points的定语从句：than（主）would otherwise have been（系）necessary（表）.
引导词than做从句主语，指more GMAT points
would otherwise have been是虚拟语气，表示“本来不……”
译文：而这本来是不必要的
Than作准关系代词时相当于：“than + the + 前面的名词 + 关系代词（who、whom、which等）。换句话说，我们可以把than视为关系代词，它可以在所引导的从句中作主语、宾语或be动词后的表语。
Ihave more books than are needed. =
Ihave more books than the books which are needed.
该句子中，than作主语，后面有谓语are needed。
Ihave more books than he needs. =
Ihave more books than the books which he needs.
该句子中，than作needs的宾语。
Sheis a better student than you are. =
Sheis a better student than the student that you are.
此处than作表语，that实际为be动词后are的表语。
Sheis a better student than you.
如果前后两个半句的谓语动词一致，前面为is，后面为are，都是be动词，故后面的are可以省略。