It is important to understand this, because when a bare infinitive is used to form a tense, it is not affected by time or subject-verb agreement.
What is a bare infinitive? - English Lessons in Brighton
I know the "it" in "it is not affected..." refers to "bare infinitive" and thus acting as a pronoun but please explain the use of "it" in "it is important..."
I'm not sure what they mean by "when a bare infinitive is used to form a tense," because infinitives don't have "tense."
Tense is always expressed by another verb (the "main verb"). Infinitives are non-finite verbs, and they always agree with subjects in pronoun form (
me, him, them, etc.), but it so happens that infinitives can appear without explicit subjects (and often do), because mentioning the subject is not important.
In your example, there's a
transformation process that makes the use of "it" obligatory. Often, for pragmatic reasons, English doesn't like infinitives appearing
first in a sentence. And so there's nothing syntactically wrong with
To understand this is important, but the message is somewhat diluted; "important" sort of gets lost there at the end. To make the sentence more expressive, we place the infinitive at the end, leaving "important" closer to the front; this is known as "extraposition:"
is important to understand this. Of course, this is not a grammatical sentence. Putting the auxiliary verb "is" (a finite verb) first means that we need to give "is" a subject, because all finite verbs go with subjects. And this function (subject of "is") is performed by "it" (a "dummy subject," as already mentioned.) Now, we have a grammatical sentence.
It is important to understand this
as I said above, infinitives often appear without explicit subjects. If you need to mention the subject of the infinitive, sometimes syntax requires the use of "for," a word (known as "complementizer") which introduces the pronoun functioning as subject of the infinitive:
It is important for us to understand this ~ It is important for me to understand this 
