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 culture

That 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
_________________

Thanks,
Ashish
EducationAisle, Bangalore

Sentence Correction Nirvana available on Amazon.in and Flipkart

Now! Preview the entire Grammar Section of Sentence Correction Nirvana at pothi.com

Yes! this was that very example i got stuck on.