[HELP] 求几种表目的的常见方法异同 (不定式,介词结构 and 状语从句)

这方面我了解一点点。

1、一般情况下，不定式比介词结构好，介词结构又比从句好。这是从简洁性上考虑，如果是固定搭配的话又除外。不过ETS往往把这个原则反过来考，也就是故意在句子中增加一些产生歧义的东西。如果你使用不定式的话，就容易产生歧义，而如果使用从句的话，就要清晰很多。因为在GMAT中，在句子内部的不定式常常被认为它的逻辑主语就是句子的主语。所以在不定式的逻辑主语和句子主语不一致的情况下，尽量使用从句。

2、一般情况下so as to后面带的不定式如果逻辑主语和句子主语一致的话，就使用这个结构，如果不一致就使用so that结构。in order that好像在GMAT里没有成为过正确答案。

以上都是简洁性的原则，而非正确性的原则，所以如果遇到ETS考查5个错误选项中挑一个错误最轻的，原则又要被打破。

Example
OG 189

189. Proponents of artificial intelligence say they will be able to make computers that can understand English and other human languages, recognize objects, and reason as an expert does—computers that will be used to diagnose equipment breakdowns, deciding whether to authorize a loan, or other purposes such as these.
(A) as an expert does—computers that will be used to diagnose equipment breakdowns, deciding whether to authorize a loan, or other purposes such as these
(B) as an expert does, which may be used for purposes such as diagnosing equipment breakdowns or deciding whether to authorize a loan
(C) like an expert—computers that will be used for such purposes as diagnosing equipment breakdowns or deciding whether to authorize a loan
(D) like an expert, the use of which would be for purposes like the diagnosis of equipment breakdowns or the decision whether or not a loan should be authorized
(E) like an expert, to be used to diagnose equipment breakdowns, deciding whether to authorize a loan or not, or the like
Answer: C

Used to do
used for sth, which is better? (In this question, it looks like "use" is not the point which ETS would like to test)

HELP HELP HELP. UP UP UP

Why only one reply. Any detail?

表目的，偶没觉得to do 比for sth更好，不过偶没从语法书上看到这两者的区别。

但to do 比for doing更好，如果for sth中的sth是动作性名词，for sth也不好。

个人觉得这样的区分太绝对化, 应该根据当时题目的逻辑意思去判断.....

不过兼雇correctiveness和effectiveness是比较好的