Cleft sentences are used to emphasize or focus attention on part of a sentence. There are several types of cleft sentences:
1) Beginning with "what" or "all" to emphasize a thing or only thing
2) Beginning with "what happens is" or "what happened was" to emphasize an event
3) Beginning with "the person who", "the place where", "the first/last time", or "the reason why"
4) Beginning with "it is/was" followed by a relative clause
The document provides examples of each type and exercises rewriting sentences using cleft structures to shift emphasis.