Intrusion test for VPs and their NP complements
Certain adverbs, such as easily, occur only as adjuncts within the VP, and hence cannot be positioned between V and its complements (since they must be sister to V').
Test sentence: Sally can easily [open the door with her credit card].
* Sally can [open easily the door with her credit card].
Sally can [open the door easily with her credit card].
Sally can [open the door with her credit card easily].
These examples show that easily the door cannot be an NP; the door must be a complement and with her credit card must be an adjunct.
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.