Cleft sentences are used to emphasize a specific part of a sentence, such as the subject, object, or adverbial phrase. They begin with "what" or "it" clauses and use forms of the verb "be". For example, "What I enjoy most is learning English" emphasizes what is enjoyed most. "It is Jenny who spends all her money on shoes" emphasizes who is spending the money. Cleft sentences can also focus on time, place, or actions using "it" clauses like "It was in London that he met his first wife."