| Definition -
(1) The use of it as a subject when the subject of the clause has been moved to a later position in the sentence, e.g. It's a pity that you're not here.
(2) The use of it as the subject in cleft sentences, e.g. It was Dave who was at the door.
(3) The use of it as the grammatical object, with the semantic object coming later in the sentence, e.g. I take it that you are irritated.