It-clauses are the most common type of cleft clause. The information that comes after it is emphasised for the listener. The emphasis in the resulting cleft sentence is on the phrase after it + be.
The clause which follows the it-clause is connected using that and it contains information that is already understood. We often omit that in informal situations when it is the object of the verb.
In the clauses that follow it + be + phrase, we can also use the same relative pronouns (who, whom, whose, which, that) and relative adverbs (where, when, why) that we normally use in defining relative clauses.
Examples:
It is my sweater that Lily wore yesterday. → (Focus on sweater)
Lily wore my sweater (not my skirt) yesterday.
It is yesterday when Lily wore my sweater. → (Focus on yesterday)
Lily wore my sweater yesterday (not today).
It is Lily who/that wore my sweater yesterday. → (Focus on Lily)
Lily (not me) wore my sweater yesterday.
If we use a personal pronoun after it + be, it will be in the object form.
It is her who/that wore my sweater yesterday.
Leonardo Da Vinci painted ‘The Last Supper’ between 1495 and 1497. (neutral sentence)
It was Leonardo Da Vinci who/that painted ‘The Last Supper’ between 1495 and 1497.
It was ‘The Last Supper’ that Leonardo Da Vinci painted between 1495 and 1497.
It was between 1495 and 1497 when Leonardo Da Vinci painted ‘The Last Supper’.
Einstein discovered the theory of relativity in Germany in 1905. (neutral sentence)
It was Einstein who/that discovered the Relativity Theory in Germany in 1905.
It was the Relativity Theory that Einstein discovered in Germany in 1905.
It was in Germany where Einstein discovered the Relativity Theory in 1905.
It was in 1905 when Einstein discovered the Relativity Theory in Germany.
What-clause
Important information can be emphasised by putting it at the end of a sentence. We put the less important information into a clause beginning with what, and open the sentence with this clause. We can then finish with the important information. The two parts of the sentence are joined with IS or WAS, since we treat the what-clause as singular.
Wh- clause (with a verb) + a form of be + emphasised information
Examples:
What helps her relax is listening to classical music.
What they hated more was his insolent remarks.
They needed money, so what they did was to ask relatives to invest in their project.
The school was in a catastrophic state, so what the teachers did was to go to the media to make this public.