The adjectival modal adverbs express to what degree of probability/plausibility the proposition expressed by the clause is true. Some typical examples of such adverbs are given in (20).

In (21) two examples are given in context; the paraphrases in the primed examples show that we are indeed dealing with modification of the full clause.

The fact that the heads of the adverbial phrases in (21) can be modified by the intensifier heel/zeer 'very' unambiguously shows that they are adjectives. However, with the exception of (zeer) beslist 'definitely' , using an intensifier is impossible with the other adverbs in (20). Nevertheless, there are other indications that these are adjectival in nature: the elements blijkbaar , schijnbaar , natuurlijk and vermoedelijk , for instance, contain the adjectivizing affixes -baar and -lijk.
      Observe that, despite the fact that the clausal adverbs in (21) can be modified by means of an intensifier, comparative formation is excluded. This is especially conspicuous in the case of the adjective waarschijnlijk , which normally does allow the comparative form. This contrast has already been illustrated in the examples in (14) in Section 8.2, sub I, which also show that on- prefixation of adverbially used waarschijnlijk is excluded.

The adjectival subjective adverbs express some subjective evaluation by the speaker of the assertion expressed by the clause. Some typical examples are given in (22).

The primeless examples in (23) provide some sentences in which these adverbs are used; their paraphrases are given in the primed examples.

      The adjectives in (22a) cannot be modified by means of an intensifier or undergo comparative formation if used subjectively. This is especially conspicuous in the case of gelukkig, which normally allows both. The adverbs are also special in that they do not allow on- prefixation. This is again conspicuous in the case of adverbial gelukkig, whose negative counterpart is not ongelukkig but ongelukkigerwijs'unfortunately'; see the discussion of (26b) below. We can illustrate all this by means of the examples in (24).

Example (24a) is ambiguous: the adjective is either interpreted as a clausal adverb, in which case it expresses relief on the part of the speaker, or it is used as a VP (manner) adverb, in which case it indicates that the fall has no serious consequences. If the adjective is modified by means of the intensifier erg 'very' , as in (24b), it can only be interpreted as a VP adverb. The same thing holds if the adjective has the comparative form or is prefixed with on-, as in (24c-d).
      Because subjective adverbs cannot be modified by an intensifier, it is difficult to establish that some of the adverbs in (22) are really adjectives. Still, the fact that onverhoopt is (in fact, must be) prefixed with the negative marker on- strongly suggests that we are indeed dealing with an adjective (cf. Section 1.3.1, sub II), and the same thing is suggested by the fact that hopelijk contains the adjectivizing affix -elijk. Another indication that we are dealing with adjectives is that some subjective clausal adverbs can be modified by the degree element genoeg . In fact, adding this element to example (24a), as in (25a), strongly favors the interpretation of the adjective as a clausal adverb. Recall, however, that the modifier genoeg 'enough' may license an infinitival om -clause if it modifies, for instance, an attributive adjective. This is not possible if we are dealing with a clausal adverb; addition of an om -clause to (25a) forces an interpretation of the adjective as a VP adverb.

      A remarkable property of genoeg is that it enables certain adjectives to be used as clausal adverbs, which otherwise cannot be used in this function; example (24d) has shown that the negative adjective ongelukkig normally cannot be interpreted as a clausal adverb, but if we add genoeg, as in (26a), this interpretation becomes readily available. Example (26b) shows that a similar effect obtains if we add the affix -erwijs to this adjective.

Diepeveen (2011) has shown that genoeg also has the ability to convert basic adjectives like interessant'interesting', which normally cannot be used adverbially, into subjective adverbs. Some examples are given in (27).

The third type of adjectival clausal adverbs are the frequency adverbs in (28), which differ from time adverbs like laat'late' in that they do not situate the event in a specific interval on the time axis, but indicate the frequency of the event.

That the frequency adverbs can function as clausal adverbs is clear from the fact illustrated in (29) that they can occur in the het is adverb zo dat ... frame. 8.2.2, sub III, will return to the frequency adverbs and show that they can be used not only as clausal adverbs, but also as VP adverbs.

      That most of the frequency adverbs in (28) are of an adjectival nature is clear from the fact that many of them can be modified by means of an intensifier or a quantifier. This is illustrated in (30).

That dagelijks/wekelijks/maandelijks are adjectival in nature is beyond doubt, given that they may occur in attributive position; cf. (31). Finally, that gewoonlijk is adjectival is plausible since it contains the adjectival affix -lijk.

In some cases, however, it is not possible to determine the categorial status of frequency adverbs, because they do not allow modification or comparative formation.

The (a)-examples in (33) show that adverbial phrases of time like laat 'late' cannot be used as sentence adverbs; Section 8.2.2, sub II, will show that adverbs like these act as VP adverbs. Nevertheless there are certain adverbial phrases of time that can sometimes act as sentence adverbs, which is illustrated in the (b)-examples by means of the adverbial PP op zondag 'on Sunday' .

The primeless examples show that this difference in function is reflected by the position of these adverbial phrases relative to the frequency adverb vaak'often': whereas the VP adverb laat'late' follows the frequency adverb, the adverbial phrase op zondag'on Sunday' precedes it. The semantic contributions of the two adverbial phrases also differ: whereas the adjective laat seems to situate the event on the time axis, the phrase op zondag expresses a restriction on the proposition: it is specifically on Sundays that Marie gets up late (not on other days).
      Adjectives cannot be readily used as clausal adverbs of time. Possible cases are vroeger'in the past', tegenwoordig'nowadays', and later'in the future'. It is not clear, however, whether these elements are really adjectives. The element vroeger in (34), for example, has a slightly different meaning than the attributively used counterpart in zijn vroegere vrouw'his former wife', and resembles the complement of the PP in We spraken over vroeger'We talked about the past'. The fact that later can be also used as the complement of a PP, as in We sparen voor later'We are saving money for later', suggests that we are actually dealing with nominal elements. We leave this for future research.

For completeness’ sake, note that the adverb vroeger in (34a) is certainly not the comparative form of the adjective vroeg'early', which is clear from the fact that vroeger cannot be replaced by the positive adjective vroeg or the superlative form het vroegst, as is shown in (35a). Example (35b) shows that all these forms are easily possible if we are dealing with a VP adverb. Similar facts can be observed for the adverb later'later'.

      Other possible cases are adverbs like zojuist'just now', straks'later/soon', gisteren'yesterday', and morgen'tomorrow'. The examples in (36) illustrate for straks that it may function both as a clause and as a VP adverb; see also the discussion of the examples in (18) from Section 8.2, sub III, which show that the position of the time adverb relative to the modal adverbs determines its construal as a clause or a VP-adverb. It is again difficult, however, to prove that we are dealing with adjectives.

The final set of adverbs that we will discuss here are the emphasizers/amplifiers. Some examples are echt'truly', absoluut'absolutely', and duidelijk'clearly'. The main function of these elements is to emphasize the truth of the assertion, or to scale upwards from an assumed norm. Some examples are given in (37). This class of adverbs is problematic as the paraphrases in the primed examples may seem somewhat unnatural and do not fully cover the meaning of the primeless examples.

The examples in (38) show that adjectives like these can also be used attributively (cf. Section 1.3.5, sub II); their adjectival status is therefore beyond doubt.

      Actually, the adverbially and attributively used adjectives in (37) and (38) are quite close in meaning in the sense that the adverbs in (37) are intimately related to the noun phrases they precede, which perhaps accounts for the fact that the paraphrases in the primed examples of (37) do not seem to fully capture the meaning of their primeless counterparts. One indication for this is that, unlike the clausal adverbs discussed earlier, the emphasizer/amplifier must be immediately left adjacent to the predicative noun phrase in (37a) and the direct objects (37b&c). This contrast is illustrated in (39), by means of the modal adverb waarschijnlijk and the emphasizer absoluut from (37b).

The value of this particular argument is perhaps not entirely clear since it could be the case that the unacceptability of (39b') is accidental: the unacceptability of the examples in (40) shows that the emphasizer absoluut is only possible if the direct object is a nonspecific indefinite noun phrase, and such noun phrases can never be placed in front of a clausal adverb; cf. *Jan zal een verhaal waarschijnlijk vertellen'Jan will probably tell a story'. Nevertheless, the contrast between the doubly-primed examples in (39) shows clearly that some adjacency requirement is at work.

Other evidence that the emphasizers/amplifiers must be left adjacent to the nominal predicate or direct object emerges if we compare the double object constructions in (41) (and the same thing can be shown on the basis of example (42c) in the next subsection). In (41a) the indirect object niemand'no one' may follow the modal adverb waarschijnlijk'probably', whereas the placement of the indirect object after the emphasizer/ amplifier absoluut'absolutely' in (41b) is impossible on the intended reading.

Given that example (41b) is acceptable if the adverb is construed as an emphasizer of the negation expressed by the pronoun niemand, perhaps we should conclude that these adverbs are actually not clausal adverbs but function as modifiers of nonspecific indefinite noun phrases. If so, the het is adverb zo dat ... frame may not be a fully reliable test for detecting clausal adverbs.

In the examples in the previous subsections, the clausal adverbs occupy a position in the middle field of the clause. With the exception of the emphasizers/amplifiers in (42c), they may also be topicalized, that is, be placed in clause-initial position. This is illustrated in (42a) for the modal and subjective adverbs and in (42b) for the frequency adverbs.

Placement of the clausal adverbs in a position following the verb(s) in clause-final position is normally not an option, although it should be observed that example (43a), with a modal/subjective adverb, is acceptable if the clause-final verbs are followed by an intonation break.

      Some of the clausal adverbs discussed above can co-occur in a single clause. Although the judgments are sometimes somewhat unclear, the preferred order of these adverbs seems to be as indicated in (44); cf. Cinque (1990).

It is not always easy to combine a subjective and a modal adjective, but if it is possible, as with vrijwel zeker'almost certainly' in (45a), the subjective clausal adverb must precede the modal one. The subjective and modal clausal adverbs precede the frequency adverbs, as is shown in (45b) and (45c), respectively. The examples in (45d) show that the frequency adverbs in turn precede the emphasizers/amplifiers.

      The examples in (46) show that the order in (44) must be preserved under topicalization, that is, topicalization of a clausal adverb across another clausal adverb is blocked. We do not give examples with echt, as this element does not allow topicalization anyhow; cf. (42c).

For completeness’ sake, note that helaas can also be used as an interjection, as in Helaas, het boek is vrijwel zeker/waarschijnlijk uitverkocht. This example differs from (46a) in that helaas does not occupy the regular clause-initial position, which is occupied by the subject het boek, and is therefore extra-clausal.