Topicalization

Topicalization is a mechanism of syntax that establishes an expression as the sentence or clause topic; in English, by having it appear at the front of the sentence or clause (as opposed to in a canonical position further to the right). Topicalization often results in a discontinuity and is thus one of a number of established discontinuity types (the other three being wh-fronting, scrambling, and extraposition). Topicalization is also used as a constituency test; an expression that can be topicalized is deemed a constituent. The topicalization of arguments in English is rare, whereas circumstantial adjuncts are often topicalized. Most languages allow topicalization, and in some languages, topicalization occurs much more frequently than in English.

Typical cases of topicalization are illustrated with the following examples:

Assuming that the a-sentences represent canonical word order, the b-sentences contain instances of topicalization. The constituent in bold is fronted in order to establish it as topic. The two former examples with a topicalized adjunct are typical, whereas the two latter examples with a topicalized object argument are comparatively rare. The appearance of the demonstrative determiners that and those is important, since without them, topicalization of an argument seems less acceptable, e.g. ?A pizza I won't eat.

Topicalization can occur across long distances, e.g.

Topicalization is similar to wh-movement insofar as the constituents that can be wh-fronted can also be topicalized, e.g.

And topicalization is similar to wh-fronting insofar as the islands and barriers to wh-fronting are also islands and barriers to topicalization, e.g.

These examples illustrate the similar behavior of topicalization and wh-fronting. Further data, which will not be produced here, could show, however, that topicalization is unlike the other two major discontinuity types, i.e. scrambling and extraposition.

...
Wikipedia