有關it's crucial to keeping文法

........ It's a tedious process, but it's crucial to keeping our records
straight.

本句的to是介系詞，意思是「對於」; 後面的keeping our records straight是動名詞片語，作為其受詞。
本句的兩個it都是代替前句的這件事，都是真正的主詞而不是虛主詞。

整句意思如下：
那是個單調冗長的過程，但這事對於保持我們紀錄正確可靠來說是至關重要的。

為什麼不是to keep呢？其實用to keep也是正確的句子，只是在這裡與前句無法連貫所以不適合。
It is crucial to keep our records straight.
= To keep our records straight is crucial.
(保持我們紀錄正確可靠是很重要的。)
單獨就這句來看是符合文法的，這個It是虛主詞，代替的是後面的大主詞to keep our records straight，而不是代替前句的那件事，所以意思與前句連貫不起來。


回到題目原句，作為真正主詞的it可以用this/that來取代，但虛主詞句型的it則不可以。



2015-03-26 21:59:57 補充：
Louis大的說法，本人礙難贊同。
在同一敘述以but連接的前後兩句中，兩個it不可有不同解讀；也就是說，兩個it只能代表同一事物，而不會一個it是實主詞，另一個it卻是虛主詞。
此外，動名詞片語前面並不以使用所有格為必要，尤其是本句的keeping our records已有our, 動作主體已經明確，不需在keeping前面重複使用our。

to keeping our records straight
=looking forward to welcoming you
=intended meaning functioning as
on going,
(Not about to)
=to+v=to+Ving (Idiom)----used only adjectivally
=participial used only adjectivally
=present participial
=ing.
Hence, present participial consisting of a participle and other words used only adjectivally is the explanation.

thanks for your helping , I think I understand it

"to" in this kind of sentence can be treated as "preposition" or "infinitive". The difference is:

2015-03-25 22:09:15 補充：
as a preposition, it emphasizes the situation is an on-going one.
as an infinitive, it emphasizes the situation has not happened yet, but shall.

2015-03-25 22:11:29 補充：
This is similar to "go shopping" and "go to work".

"go shopping" emphasizes "shopping" is on-going.
"go to work" indicates that one is about to work, not working yet.

2015-03-25 22:13:01 補充：
"to" can have different function, you shall based upon the intended meaning of the sentence to determine which function to use. Don't always think "to + V"!

2015-03-25 22:19:06 補充：
Great! This kind of explanation is not seen in grammar book or taught by teacher, however, you need to use your mind to sense and understand this kind of minute (tiny) difference in using English.