Study materials
Three sentences about the same event. The fact is identical; a different word is under the spotlight each time.
The auditors raised the issue in March.
It was the auditors who raised the issue.
It was in March that the auditors raised it.
English cannot raise its voice on paper. Instead it cuts the sentence in two and puts the word you mean where it cannot be missed.
The it-structure: focus straight after be
The pattern is fixed: it plus a form of be, then the focused element, then the rest with who or that.
It was the auditors who raised the issue.
It was in March that the policy changed.
It isn't the cost that worries us.
Almost any element can be focused: a person, a time, a place, a reason, a circumstance. Only the action itself cannot; that has a structure of its own.
The form of be is nearly always past or present and does not agree with the focused word: It was the auditors, not It were the auditors.
What it is for in a text
This structure nearly always corrects an assumption. It implies that the listener thought otherwise, which is why in conversation it sounds slightly combative.
It wasn't the manager who signed it. It was the deputy.
That is why it is out of place in neutral description: the reader was not arguing, and you are already correcting them.
The mistakes everyone makes
The first is agreeing be with the focused word.
It were the auditors who raised it. → It was the auditors who raised it.
The second is taking which for a person.
It was Maya which solved it. → It was Maya who solved it.
The third is focusing an action with it.
It was rewrite the section that she did. → What she did was rewrite the section.
All three are caught by one question. What exactly am I focusing, and did the reader really think otherwise?
Try it right now
Take a neutral sentence about your working week and focus, in turn, the person, the time and the place.
Then read all three versions and check whether in each you are mentally arguing with somebody.