for people still think of it as a day for giving thanks.补充原句：Thanksgiving Day is also a day for students to give thanks to parents and teachers,for people still think of it as a day for giving thanks.
网友回答
for people still think of it中的for本就带有对..来说的意味,加to的话语义上就不太通顺.as是作为的意思吧.后面的for表示目的.是介词,自然要加ing形式~
======以下答案可供参考======
供参考答案1:
for people still think of it as a day for giving thanks
这句的意思是:因为人们仍然认为那是一个用来感恩的日子!是一般的叙述现状,当然不用不定式;
as在这里的意思是作为,当作的意思;
for是个介词,后面当然要用动名词形式!
供参考答案2:
think of ... as ...是一个固定搭配，表示”认为。。。是“”把。。。看作/当成“。
think of ... as ＝look on ...as =have...as
for 是介词 后面接动词当然要用动词的ing形式哦。因为只有名词和代词才可以做介词宾语。这里giving做动名词。