it is most often used as a pronoun referring to a thing previously mentioned or easily identified, or as an abstract subject placed in opposition to you and I. It can also be used as a placeholder for a known number or a missing term, for example, 'I have two apples; if you have two apples, then we have four all together -- it’s simple math.' It can also be used to refer to an entire situation, such as 'It has been raining all day,' or to refer to an unidentified person, as in, 'It must be nice.'
1. It was a hard decision, but ultimately the right one.
2. It appears that the train is running late again.
3. It was nice of you to do that for him.
It is important to remember that it should always refer to a known or previously mentioned thing. It should not be used to refer to a person, so avoid phrases like 'I love John, it is a good man.'