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 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 no 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．
参考资料：Nickolas83