While the classical theory recognizes four types of null DPs (DP-trace, WH-trace, PRO, and
pro), recent research has found evidence for null DPs that don't appear to fit the classical model such as the distinction of null subjects and null objects.
The classical theory In the classical theory model, empty (or null) DPs can be broken down into four main types: DP-trace, WH-trace, PRO, and
pro. Each appears in a specific environment, and is further differentiated by two
binding features: the
anaphoric feature [a] and the
pronominal feature [p]. The four possible interactions of plus or minus values for these features yield the four types of null DPs. In the table, [+a] means that the particular element must be bound within its governing category. [+p] means that the empty category is taking the place of an overt pronoun. Having a negative value for a specific feature indicates that a particular type of null DP is
not subject to the requirements of the feature. Not all empty categories enter the derivation of a sentence at the same point. Both DP-trace and WH-trace, as well as all the null heads, are only generated as the result of
movement operations. "-trace" refers to the position in the sentence that holds syntactic content in the
deep structure, but that has undergone movement so that it is not present at the
surface structure. Conversely, both "PRO" and "
pro" are not the result of movement and must be generated in the deep structure. PRO is a universal lexical element, it is said to be able to occur in every language, in an environment with a non-finite embedded clause. such as in the example below: This example does not use PRO, but instead, uses an overt pronoun ("you") in the specifier position of the embedded non-finite clause: 1a)
Hei would like youj to stay. This example does use PRO, because instead of an overt pronoun, there is an empty category which is
co-referenced with "He", appearing in the specifier position of the non-finite embedded clause: 1b)
Hei would like PROi to stay. The example tree to the right is the tree structure for this sentence, [Hei would like PROi to stay], and shows PRO surfacing in the specifier position of the TP in the embedded clause, and co-referenced to (referring to the same being as) the subject of the matrix clause. We can interpret this as the DP subject [He] having control over PRO. In other words, the meaning of PRO is determined by the meaning of DP [He], as they are co-referenced. This is an example of a subject control construction, where the pronominal subject [He] is selected for by both the main verb [like] and the embedded infinitive verb [stay], thus forcing the introduction of an unpronounced
lexical item (PRO) at the subject of the embedded clause, in order to fulfil the selectional requirements of both verbs. Alternatively, we see object control constructions when the object of the sentence controls the meaning of PRO. However, while the meaning of PRO can be determined by its controller (here, the subject of the matrix clause), it does not have to be. PRO can either be controlled ("
obligatory control") or uncontrolled ("
optional control"). The realization that PRO does not behave exactly like an R-Expression, an anaphor, or a pronoun (it is in fact, simultaneously an anaphor and a pronoun) led to the conclusion that it must be a category in and of itself. It can sometimes be bound, is sometimes co-referenced in the sentence, and does not fit into
binding theory.
pro (little pro) "Little pro" occurs in a subject position of a
finite clause and has case. The DP is ‘dropped’ from a sentence if its reference can be recovered from the context;
"pro" is the silent counterpart of an overt pronoun. Spanish is an example of a language with rich subject-verb
morphology that can allow null subjects. The agreement-marking on the verb in Spanish allows the subject to be identified even if the subject is absent from the spoken form of the sentence. This does not happen in English because the agreement-markings in English are not sufficient for a listener to be able to deduce the meaning of a missing referent. Chinese is an example of a
pro-drop language, where both subjects and objects can be dropped from the pronounced part of
finite sentences, and their meaning remains clear from the context. In pro-drop languages, the covert "pro" is allowed to replace all overt pronouns, resulting in the grammaticality of sentences that do not have a subject nor object that is overtly pronounced: This example illustrates how a
Chinese question might be asked with "Zhangsan" as the subject and "Lisi" as the object: {{interlinear|number=3a) Below is an example of a response to the question above. Both subject and object are optionally pronounced categories. The meaning of the sentence can be easily recovered, even though the pronouns are dropped. (Round brackets indicate an optional element.) When this happens, a movement rule is initiated, and the structure is altered so that we hear the DP overtly pronounced in the position of the sentence which it has been moved to; a DP-trace is an empty category that appears at the original spot (the underlying position) of the DP, and stands for the syntactic space in the tree that the DP previously occupied. DP-trace is found in
complementary distribution to PRO. and leaves behind a DP-trace (tDP) in the original position of the DP. You can use the position of the DP-trace to identify where the DP is introduced in the
underlying structure.
WH-trace (tWH) File:WH-trace.png|thumb|This example shows the movement of "WH" DP [who] from the complement position of the verb to the specifier position of CP. DPs can move for another reason: in the case of Wh-questions. In English, these are questions that begin with (e.g. who/whom, what, when, where, why, which, and how); words that serve the same function in other languages do not necessarily begin with , but are still treated as “Wh-items” under this framework. The responses to these questions cannot be yes or no; they must be answered using informative phrases. Wh-items undergo Wh-movement to the specifier of CP, leaving a Wh-trace (tWH) in its original position. Just like for DP-movement, this movement is the result of feature checking, this time, to check the [+WH] feature in C. To form a Wh-question in the example below, the DP [who] moves to the specifier of the CP position, leaving a Wh-trace in its original position. Due to the
extended projection principle, there is DP movement to the specifier of TP position. There is also T to C movement, with the addition of
Do-support. These additional movement operations are not shown in the given example, for simplicity. Example 5: Underlying order of words in the sentence “Who did Lucy see?” (Square brackets throughout example 5 indicate an empty category.) 5a)
[ ] did Lucy see who Spoken form of the sentence "Who did Lucy see?" 5b)
Who did Lucy see [ tWH ]? • Square brackets throughout example 5 indicate an empty category. • You can see where "Who" was in the initial word order by where the WH-Trace appears in the spoken form. The tree to the right illustrates this example of WH-trace. Initially, the sentence is "[CP] did Lucy see who,” which has an empty specifier position of CP, as indicated by square brackets. After the Wh-item [who] is relocated to the specifier position of CP, the empty position is left at the end, in the original position of [who]. What is left in its place is the WH-trace. A special relationship holds between the WH-item and the complementizer of a sentence: 6a)
[DP The person [CP who ØC [TPlikes Max] is here. 6b)
[DP The person [CP tWH that [TPlikes Max] is here. 6c)
* [DP The person [CP tWH ØC [TPlikes Max] is here. In this example, the complementizer or the WH-item can have null categories, and one or the other may show up as null. However, they cannot both be null when the WH-item is the subject. An important note to remember is that DP-trace and WH-trace are the result of movement operations, while "pro" and "PRO" must be base generated. Unlike
pro (little pro), variable empty objects are R-expressions and must respect Principle C of
Binding Theory. The following is an example of a null variable object construction in
Portuguese: {{interlinear|number=8) == Null heads ==