Cleft Sentences
A declarative sentence, such as David studied English at Oxford can be reformulated as:
It was David who studied English at Oxford
This is called a CLEFT SENTENCE because the original sentence has been divided (or "cleft") into two clauses: It was David and who studied English at Oxford. Cleft sentences focus on one constituent of the original sentence, placing it after it was (or it is). Here we have focussed on the Subject David, but we could also focus on the Direct Object English:
It was English that David studied at Oxford
or on the Adjunct at Oxford
It was at Oxford that David studied English
Cleft constructions, then, exhibit the pattern:
It + be + focus + clause