There is little agreement in the literature on the question as to whether we should distinguish a separate category of adverbs. Proponents of the position that we should, e.g., Haeseryn et al. (1997:451) and Ernst (2002:8), define this presumed word class as consisting of lexical elements (and perhaps phrases) that can only function as adverbials. Nevertheless, it is customary for grammars to include elements in the set of adverbs that do not satisfy this criterion. Haeseryn et al. (1997:454), for instance, list the form morgen'tomorrow' in examples such as (137a) as an adverb despite the fact that example (137b) shows that it can occur as the complement of a preposition. Because complements of prepositions are typically nominal, the examples in (137) should lead us to the conclusion that morgen is not an adverb but a noun.

Another criterion that is sometimes used is that adverbs are invariant in form. This can be found in Haeseryn et al. (1997:451) but the same grammar simultaneously claims that certain adverbs such as the manner adverb snel'fast' and the frequentative adverb vaak'often' in (138) do allow comparative and superlative formation. Given this, there is no clear reason not to call these lexical items adjectives.

Haeseryn et al. (1997:454) contrast “spurious” adverbs such as snel'quick' and vaak'often' in (138) with true adverbs, which are invariant in form. They illustrate these “true” adverbs by means of the locational elements buiten'outside' and hier'here' in (139a). However, the fact that the form of buiten is invariant also follows if we assume that it is actually a preposition, just as buiten in Jan speelt graag buiten het gebouw'Jan likes to play outside the building'. Furthermore, the fact that hier'here' in example (139b) is invariant is due to the fact that we are dealing with a proform: in this respect, locational pro-forms simply behave like pronouns such as hij'he' and hem'him'. That we should not consider the elements buiten and hier to be adverbs is also supported by the fact that they can also be used in other syntactic functions; this is illustrated in the primed examples in (139) by means of the verb zetten'to put', which obligatorily selects a complementive.

Since lexical items used as adverbial phrases have no characteristic morphological features either, there is hardly any reason for assuming a separate category of adverbs for Dutch; they can normally be analyzed as a noun, as morgen'tomorrow' in (137a), an adjective, as snel'fast' and vaak'often' in (138), or a preposition, as buiten'outside' in (139a), while many of the remaining cases are pro-forms. We therefore provisionally conclude that the category of adverbs does not exist (although we will keep using this notion for convenience); we refer the reader to Section N8.2, A8, and P1.2.4 for more relevant discussion. Nevertheless, the following two sections will occasionally point out forms for which it is not easy and perhaps even impossible to determine to what category they belong: the existence of these forms shows that the question as to whether we need a separate category of adverbs is still not fully answered.