There is little agreement in the literature about whether we should distinguish a separate category of adverbs. Proponents of the position that we should, like Haeseryn et al. (1997:451) and Ernst (2002:8), define this putative word class as consisting of lexical items (and perhaps phrases) that can only function as adverbials. Nevertheless, it is common for grammars to include items in the set of adverbs that do not satisfy this criterion. For example, Haeseryn et al. (1997:454) lists the form morgentomorrow in examples such as (141a) as an adverb, even though example (141b) shows that it can occur as the complement of a preposition. Because complements of prepositions are typically nominal, the examples in (141) should lead us to conclude that morgen is not an adverb but a noun.

Another criterion sometimes used is that adverbs are invariant in form. This can be found in Haeseryn et al. (1997:451), but the same grammar also claims that certain adverbs, like the manner adverb snelfast and the frequentative adverb vaakoften in (142), allow comparative and superlative formation. Given this, there is no obvious reason not to call these lexical items adjectives.

Haeseryn et al. (1997:454) contrasts “spurious” adverbs like snelquick and vaakoften in (142) with “true” adverbs like the locational elements buitenoutside and hierhere in (143a), which are invariant in form. However, the fact that the form of buiten is invariant also follows if we assume that it is an (intransitively used) preposition, as buiten in Jan speelt graag buiten het gebouwJan likes to play outside the building. And the fact that hierhere in example (143b) is invariant is due to the fact that we are dealing with a proform: in this respect, locational proforms simply behave like pronouns such as hijhe and hemhim. That we should not consider the elements buiten and hier as adverbs is also supported by the fact that they can be used in syntactic functions other than adverbial; this is illustrated in the primed examples in (143) with the verb zettento put, which obligatorily selects a complementive.

Since the lexical items used as adverbial phrases have no characteristic morphological features either, there is little reason to assume a separate category of adverbs for Dutch; they can usually be analyzed as nouns, such as morgentomorrow in (141a), adjectives, like snelfast and vaakoften in (142), or prepositions, such as buitenoutside in (143a), while many of the remaining cases are proforms. We therefore provisionally conclude that the category of adverbs does not exist (although we occasionally use the term for convenience); we refer the reader to Sections N21.2, A30, and P32.2.4 for more relevant discussion. Nevertheless, in the following sections we will occasionally point out forms for which it is not easy, and perhaps even impossible, to determine which category they belong to: the existence of these forms shows that the question of whether we need a separate category of adverbs is still not fully answered.