Definition
Cleft sentences are constructed to give prominence or focus to some element such as a noun, pronoun, or interrogative. In a cleft sentence, the focal element is the subject and the embedded clause is a predicate complement Cleft sentences involve the splitting of a simple clause into a focal element and an embedded clause, in this case a predicate complement, from which the focal element is extracted, in this case the subject. (Suttles, 2003, p. 110)
Examples

References
Suttles, W. (2004). Musqueum reference grammar. UBC Press. SFU Student Access.