"It" is a pronoun. As a personal pronoun it refers back to something that has already been mentioned.

是代词，在用作人称代词时指代前文提过的某事。

youdao

As has already been mentioned, they were supported by an airlift of hope, and we are so honored to be joined by Colonel Halvorsen, 92 years old -- the original "candy bomber. ""

正如刚才提到的，柏林人得到了承载着希望的空运支持，而我们很荣幸92 岁高龄的哈尔沃森上校今天来到这里—— 他是当年的“糖果投弹手”。

youdao

One such model has been already mentioned in this article: the COCOMO II equation that links effort (cost) to personnel, environment, quality, size, and process.

在本文中我们已经提到了一个模型：COCOMO II等式，它把工作量(成本)与人员，环境，质量，规模和过程联系起来。

youdao

First, as has already been mentioned, we're moving ahead with our landmark trade agreement.

WHITEHOUSE: President Obama and President Santos Hold a Press Conference | The White House

As has already been mentioned, their stars now grace this memorial wall.

WHITEHOUSE: President Obama Thanks the Intelligence Community

As has been already mentioned, when guidance is conducted by a course in career development, it is not unusual to have the course taught by regular classroom teachers who have had special preparation to teach the course.

UNESCO: GUIDANCE