any problem in grammar? should I need a preposition?

是否需要用preposition, 或者用咩preposition, 係取決於:

動詞本身
你的句子想表達甚麼意思 (有機會因情況而異)
「為某些人的利益著想」 當中的「為」字就正是"for".
http://edition.cnn.com/2008/LIVING/worklife/06/25/cb.first.job/index.html
實例: 12 things to consider for your first job (i.e. You have to consider these things for your first job)
________________________________________________________
"to" 和"for"的確有意思或用法相近的地方, 兩者都有「前往某地方/ 向某地點進發」的意思 e.g. She usually goes to school by bus. After his retirement, he headed for England. 而"to"和"for"後面都可以係「某些對象/人/物」等object.
http://www.ldoceonline.com/dictionary/to_2

圖片參考：http://www.ldoceonline.com/images/entry/pronsentencea.gif
He sent presents to the children.

圖片參考：http://www.ldoceonline.com/images/entry/pronsentencea.gif
Don't show these letters to anyone else.



圖片參考：http://www.ldoceonline.com/images/entry/pronsentencea.gif
She stood up and walked to the window.


圖片參考：http://www.ldoceonline.com/images/entry/pronsentencea.gif
sending a spaceship to Mars.
....................................

http://www.ldoceonline.com/dictionary/for_1



圖片參考：http://www.ldoceonline.com/images/entry/pronsentencea.gif
I've got a present for you.

圖片參考：http://www.ldoceonline.com/images/entry/pronsentencea.gif
Someone left a message for Vicky.



圖片參考：http://www.ldoceonline.com/images/entry/pronsentencea.gif
I looked after the kids for them.

圖片參考：http://www.ldoceonline.com/images/entry/pronsentencea.gif
Let me carry that bag for you.

________________________________________________________

但是, 就你的例子 (你為他人的利益著想), 你跟對方可能根本完全無見過面, 無接觸, 或者係未必有一件實物交與對方, 所以沒有指 "to" (前往) 的意思, 而係偏向「為著某個對象/ 目標做事」(for) 的意思較多.


圖片參考：http://tw.yimg.com/i/tw/ugc/rte/smiley_1.gif
peace

2008-10-17 20:03:08 補充：
「無見過面, 無接觸」 的情況可以係:

例如, 那些人係寄信, send email, 經第三者(可以係議員/ 代表)提出要求(意見)等等.

I prefer 'for those'