Section 8.3.2 has shown that most VP adverbials can appear in different forms: they can generally appear in adjectival or prepositional form, and in some cases they can even be nominal or clausal. Since VP adverbials are typically phrasal, they can be formed productively. This is also true of locational, temporal, and contingency clause adverbials: they are not substantially different from their counterparts that function as VP adverbials. However, many clause adverbials are quite restricted in form, as can be seen from the fact that they often form a closed class of lexical items, and it is therefore not surprising that these items are often thought of as belonging to a category of adverbs. Consider again the subclasses of clause adverbials in (174), taken from Section 8.2.2.

Polarity adverbials clearly form a closed class, which contains only the negative element nietnot and the affirmative element wel. These are usually considered to be adverbs, as it is not easy to find decisive arguments for placing them in one of the four major lexical categories.

The same goes for the focus particles in (174b): they form a relatively small set, and again it is difficult, if not impossible, to show that they belong to one of the major lexical categories.

The categorial status of some of the aspectual adverbs is not difficult to establish: habitual gewoonlijkusually and frequentative vaakoften are clearly adjectival, while drie maalthree times is clearly nominal. However, there are also many cases where the category is less easy to determine; specific examples are continuative nog (steeds)still, terminative niet meerno longer, iterative weeragain, and alalready.

Clause-degree adverbials again form a more or less closed class: bijnanearly; amperhardly, haastnearly. Some of these elements can also be used as degree modifiers of adjectives, but again it is difficult to determine whether they belong to one of the major lexical classes.

This is easier for the adverbials in (175) expressing propositional modality, since they are often recognizable as adjectives by their morphological form: many cases are derived by the suffixes -(e)lijk and -baar, and some can be preceded by the negative prefix on-.

However, the adjectives in (175) often have restricted behavior when used adverbially; e.g. the examples in (176) show that while it is easy to question waarschijnlijk when it is used as a complementive, this is not possible when it is used adverbially. However, it does not seem that this restriction is syntactic in nature: modal adverbials are often speaker-oriented in the sense that they give the speaker’s evaluation, and this may make it unlikely that the speaker will question such a modal.

It is more difficult to explain why adverbially used adjectives have restrictions on modification that are not found for their attributively/predicatively used counterparts. For instance, while the primeless examples in (176) show that waarschijnlijk can be modified by the degree modifier zeervery regardless of its syntactic function, the degree modifiers ergvery and vrijfairly in the (a)-examples in (177) lead to marked results when waarschijnlijk is adverbial, and the (b)-examples show that similar observations can be made with respect to comparative formation. Finally, the (c)-examples show that adverbial waarschijnlijk is also more restricted than its attributive and predicative counterparts in that it does not allow for on- prefixation.

A special case worth mentioning is soms, which usually occurs as a frequency adverbial meaning “sometimes”, but can also occur as an epistemic modal in yes/no questions.

Subject-oriented adverbials are unambiguously adjectival, but are nevertheless severely limited in form: they are usually followed by the modifying element genoegenough, formed by the unproductive deadjectival suffix -elijk, or involve other less productive formations like domwegstupidly and botwegbluntly; cf. Diepeveen (2012) for a relevant discussion of these deadjectival forms (as well as some of the other complex adverbial forms mentioned later in this section).

Subjective adverbials are probably also adjectival, as shown by the fact that gelukkigfortunately and vanzelfsprekendobviously/self-evidently are common adjectives. This position is further supported by the fact that these adverbials are sometimes modified by the element genoeg or formed by the deadjectival suffixes -erwijs and -lijk: cf. jammer genoegunfortunately, begrijpelijkerwijsunderstandably, and hopelijkhopefully. However, it is difficult to demonstrate this for the form helaasunfortunately. Note that these adverbials are speaker-oriented in the sense that they give the speaker’s evaluation, and it is therefore understandable that these adverbials cannot be questioned.

Point-of-view adverbials are prototypically PPs headed by volgensaccording to; other cases are the PP naar mijn meningin my opinion and the formulaic case-marked form mijns inziensin my view.

We can be brief about the spatiotemporal clause adverbials, since they exhibit the same freedom in form as their counterparts functioning as VP adverbials; cf. Section 8.3.2, sub III.

More or less the same applies to contingency adverbials; we can only add to the discussion in Section 8.3.2, sub IV, that conditional adverbials are typically expressed by a clause introduced by a subordinator like indienin the event of, mitsprovided that, or tenzijunless. Note also that conditional clauses introduced by mits/tenzij can only be used in the right periphery of the clause. Occasionally we also find (deictic) conditional PPs: cf. Onder deze voorwaarde mag hij komenHe may come on this condition.

The prototypical case of a conditional clause is probably a clause introduced by alsif. However, there is reason to think that such conditional clauses are at least sometimes in a main-clause external position and therefore should not be analyzed as adverbials. This is quite clear in (181a), where the first position of the main clause is filled by the resumptive proform danthen. Example (181b) also shows that such conditional clauses are special in that they alternate with clauses with the finite verb in first position; cf. Section 10.3.2 for a detailed discussion of examples such as (181) as well as various related constructions.

Section 8.2.2, sub XI, has already shown that domain adverbials are usually adjectival, although it is also quite common to use phrases headed by a past/passive participle. This is illustrated again in (182).

Speech-act related adverbials such as eerlijk gezegdhonestly are usually expressed by phrases consisting of a participle verb modified by a manner adverb; omitting the past/passive participle will give rise to an unacceptable result.

Finally, conjunctive adverbials like echterhowever and derhalvetherefore again seem to form a more or less closed set of elements; cf. Section 8.2.2, sub XII, for a representative sample of such adverbials.

This subsection has shown that clause adverbials generally exhibit less variation in form than VP adverbials, with the exception of spatiotemporal and contingency adverbials. Moreover, clause adverbials also seem to be subject to various idiosyncratic constraints and tend to belong to lexically closed classes, which has motivated earlier claims in the literature that a separate category of adverbs should be recognized; cf. Section 8.3.1 for discussion.