Two superficially different sentences are shown in these examples: Charlie broke the window. The window was broken by Charlie. In traditional grammar, the first is called an active sentence, focusing on what Charlie did, and the second is a passive sentence, focusing on The window and what happened Syntax 97 to it. The distinction between them is a difference in their surface structure, that is, the different syntactic forms they have as individual English sentences. However, this superficial difference in form disguises the fact that the two sentences are very closely related, even identical, at some less superficial level.

This other “underlying” level, where the basic components (Noun Phrase + Verb + Noun Phrase) shared by the two sentences can be represented, is called their deep structure. The deep structure is an abstract level of structural organization in which all the elements determining structural interpretation are represented. That same deep structure can be the source of many other surface structures such as It was Charlie who broke the window and Was the window broken by Charlie?. In short, the grammar must be capable of showing how a single underlying abstract representation can become different surface structures.