Hi Sahil, the use of
it at the beginning of a clause almost always represents the case where the pronoun
it is used
before the antecedent. For example:
It is difficult to thrive in new culture.
Here
it is referring to the infinitive phrase
to thrive in new cultureThat at the beginning of a sentence can be used when a
clause is the
subject of the sentence. For example:
That people behave in uncertain ways is well known.
The subject of this sentence is the clause
that people behave in uncertain ways.
Note that when a
clause is the subject of the sentence, the subject is always considered
singular.
If you have specific official examples in this regard, please post them here for further discussion.
p.s. Our book
EducationAisle Sentence Correction Nirvana discusses
this usage of it, its application and examples in significant detail. If you or someone is interested, PM me your email-id; I can mail the corresponding section