Proform test: VPs
Proform: do so (replaces a V')
Test sentence: Sally will open the door with her credit card on Monday.
and John will do so too (open the door with her credit card on Monday is a V')
and John will do so on Tuesday (open the door with her credit card is a V')
and John will do so with his key on Tuesday (open the door is a V')
* and John will do so the window (open alone is not a V')
Some verbs (e.g. give, put) have two complements.
The do so-test shows that VPs headed by such verbs cannot have an X' structure in which one complement is sister to V° and another is sister to V' (since that would make it an adjunct):
John would give his key to the neighbors on Monday
* but Sally would do so to her cousin on Tuesday
One possibility is that such two-complement structures are ternary- (rather than binary-) branching. With such a structure, [give his key to the neighbors] would be the lowest V', and thus account for the ungrammatical do so replacement above.
Example: VP [give the key to the neighbors]
In the text above we refer to constituents of the type [determiner + noun] (e.g. the door) as NPs. In a more recent analysis, determiners are considered to be the head (D°) of a determiner phrase (DP), and the noun functions as its NP-complement.
For further information on DPs click here.