as to 和 as for 都是“关于”的意思,有什么用法上的区别吗?

as to 和 as for 都是“关于”的意思,有什么用法上的区别吗?

as for (至于,就……方面说)和 as to (至于；关于)这两个复合介词在含义上相当接近,但在用法上差别很多.具体差别如下：
1．as for
1)用以转换话题：在有前文的情况下as for总是出现在下文的句首,用来引出另外一方或者新的谈话内容,但与前文内容也有一定的联系,起到补充前文又突出后文的作用,使之与前文形成对照,译作“至于”.例如：
①You van have a bed；as for him,he'll have to sleep on the floor．
②Much pasture land is underwater；and as for the grain,most of that has been ruined．
2)用以表现态度：as for还可以用来表示讲话人对某人或某物持有“轻蔑”或“冷淡”的态度,甚至有憎恶之感,讲话时要重读引出的词语并在其后稍作停顿,以示强调,口气上带有嘲讽意味,译作汉语的“至于……嘛”.例如：
①As for you,I never want to see you here in my home again．
②As for his songs,I suppose you've read the sarcastic reviews !
2．as to
1)表示涉及某点：as to常放于句中,表示“关于……方面”、“有关……之事”,但为了突出要提及的人或事之时,也可将as to置于句首,以示强调.但只涉及内容,而不涉及讲话人的态度.例如：
①His wife,an old teacher,has n0 complaint as to salary．
②As to your ability to overfulfil the target,I never had the slightest doubt．
2)用以重提旧事：as to引出的内容都是前文曾经提到过的或者谈话对方都有所了解的,表示“说到”之义.,例如：
①As to doing that,I haven't decided yet．
②As to the question you raised in your last letter,I think that it is
unanswerable．
3)用于提出问题：as to必须位于句中,前面出现的大都是表达某种心理活动的动词,形容词或名词,其后引接出wh-clause或who-to-v结构,表示“在……的问题上”、“就……问题来讲”,所引出的问题都有很强的针对性.但有人认为这里的罄to显得多余,去之亦通.所以,有时也不必译出.例如：
①There is some doubt as to whether the proposal is practicable．
②I enquired of the clerk as to which documents were needed．
4．引出做事的依据：as to还有“按照”、“根据”的含义,而as for则无此义.例如：
①The shoes are correctly placed on the shelf as to size and colour．
②The items on the agenda are listed as to order of importance．
综上所述,as for 和 as to在确切含义的表达和具体用法上都是有很大差别的,不能将二者混为一谈.
as for通常总是放在句首
As for that,we shall see later．至于那件事,往后我们便见分晓了
对于突然想到的事情或临时提起的问题,以使用as for为宜,并可引导动名词短语.
例如：Much pasture land is under water；and as for the grain,most of it has been ruined．许多牧场被水淹了；至于粮食,大部分也毁了.（粮食是临时想到的,用as for来引导比较合适.）