A cleft sentence is a particular sentence structure in the English language which splits a simple sentence into one with a main clause and a subordinate clause. By using this technique, a writer can emphasize one part of a sentence much like a speaker can by intoning certain words. In a cleft sentence, the word "it" is often used as the starting point, followed by a form of the word "to be" and the main focus of the sentence that the writer wishes to emphasize. Another way to form this sentence is to begin with a relative clause begun by an interrogative word to set up the main emphasis of the sentence.
When someone goes about giving a speech to a crowd, he or she can control the points of emphasis with the tone of his or her voice. By bringing a certain amount of extra authority to certain words, a speaker can let the listener know what he or she wants them to remember. Writers usually don't have that luxury, but they can show emphasis with the use of a cleft sentence.
As an example of such a sentence, consider first the simple sentence, "I want to study history in school." Taken as it is, that sentence can be emphasized in several different ways by a speaker. By making it a cleft sentence, the writer can pinpoint the emphasis. For example, he could say, "It is history that I want to study in school." That leaves no doubt where the emphasis should be placed.
Another way that a cleft sentence can be formed is through the use of a relative clause. These clauses generally start with an interrogative word like "who" or "what." One such example would be a transformation of the simple sentence, "I want to travel to Italy in the spring." It can be changed to read, "Where I really want to travel is Italy in the spring." This sentence puts the emphasis on Italy.
By shifting the words and clauses, the writer of a cleft sentence can shift the emphasis anywhere he or she wants. For example, the sentence from the previous paragraph can be rephrased to say, "When I want to travel to Italy is the spring." It can also be changed to the sentence, "What I want to do is travel to Italy in the spring." Much like a speaker shifts his intonation, these subtle shifts in wording can signify changing emphasis in a sentence.