Omission test for complements and adjuncts

I. Basis for test

Complements are obligatory and adjuncts are optional, so the latter but not the former should be omissible.

Test sentence: Sally can easily [open the door with her credit card].

Sally can easily [open the door].
* Sally can easily [open with her credit card].

II. Complication

(a) VPs: complements can be omitted in certain contexts; e.g. recipe contexts:
Bake for fifteen minutes.

(b) NPs, APs, PPs: there are complement-taking and complementless versions of the same word:
father vs. father of the bride
envious vs. envious of John
fall down vs. fall down the stairs

(a) If an XP associated with a head can be omitted, this does not prove that it is an adjunct.
(b) If an XP associated with a head cannot be omitted, this does prove that it is a complement.

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.

Exercises on the omission test