A cleft sentence is made by separating a single clause into two clauses, one main and one subordinate. Cleft sentences are useful to change emphasis; for example, compare ‘You stole the money' and ‘It was you who stole the money, not him'.
‘It was me who broke the vase' is a cleft sentence made up of two clauses. A single clause version would be ‘I broke the vase'.
In the classroom
Sentence transformations are a form of controlled practice of cleft sentences, for example:
Pat broke the window.
Was: ...It was Pat who broke the window.