_____ the students,it is very important to know how to study.A To B For 答案是For,为什么To 不行

for 是引导动词不定式的逻辑主语，
可以改为 it is very important for the students to know...

(A) for :
(1)He prepared lunch for us.[给...受益者]
(2)He headed off for town.[指...目的地]
(3)He spoke for all the members.[代表...]
(4)He paid ten dollars for a ticket.[付款的...目的/物件]
(B) to :
(1)He turned to me and said...[朝]
(2) He went to the city.[指...方向]
(3) Their faces were pressed to the windows.[接触]
(4)We stood face to face.[正面]
(5)He was waiting for an answer to my letter.[关于]
(6)They worked from nine to five.[至]
(7)I'd like to go.[用在动词前...]
(8) Refer to a/the dictionary...[补语动词的关系].

to sb表示对某人直接影响,比如食物对某人不好就用to
for表示从意义、价值等简介角度来说,如对某人而言是重要的就用for

for , 变一下句子就懂了： It is very important (for the students )to know how to study.