The set of main verbs can be characterized semantically by the fact that they function as n-place predicates that denote certain states of affairs; see Section 1.2.3 for a more detailed discussion of the latter notion, which is a cover term for states and several types of events.

This semantic property is reflected syntactically by the fact that main verbs normally function as argument-taking heads of clauses. That main verbs function as the head of their clause is clear from the fact that they are normally indispensable; the primeless examples in (13) would normally not be recognizable as clauses without the verb. The arguments of the verbs are of course needed in order to express a proposition, but they are not as indispensable as the verb. This will be clear from the fact that the imperatives in the primed examples are completely acceptable despite the fact that the arguments of the verb remain implicit.

That main verbs function as the semantic heads of clauses is also clear from the fact that clauses contain at most a single main verb; sentences that contain more than one main verb are normally construed as involving more than one clause. The examples in (14), for instance, are cases of embedding: the bracketed phrases function as embedded direct object clauses of the matrix verbs vermoeden'to suspect' and vertellen'to tell'.

      Given that copular verbs can occur as the single verb of a clause, they are normally also considered main verbs even though they do not meet the semantic criterion of denoting states of affairs; they are not n-place predicates on a par with the predicates in (12) but instead resemble the non-main verbs discussed in the next subsection in that they express specific temporal, aspectual or modal notions. The copular verb zijn in (15a), for example, situates the state expressed by the proposition ill(Jan) in a particular position on the time axis by carrying a tense marking ±past: the present tense marking on is in (15a) expresses that the state of Jan being ill holds at the speech time. The copulas worden and blijven in (15b) in addition express aspectual information: worden'to become' is mutative in that it indicates that Jan is in the process of obtaining the state of being ill; blijven'to stay' is in a sense the opposite of worden in that it expresses that the state of Jan being ill continues to exist. Copular verbs like lijken'to appear' or blijken'to turn out' in (15c) are modal in nature given that these examples specify the attitude of the speaker with respect to the truth value of the proposition.

Although the distinction between main and non-main verbs seems to be relatively clear-cut, it is not easy to provide an operational definition of the distinction, so it is not surprising that grammars on Dutch may differ in where they draw the dividing line between the two categories. Like many other Dutch grammars, Haeseryn et al. (1997:46) assume that main verbs are predicative, that is, "express the core meaning of the verbal complex", whereas non-main verbs function as modifiers that provide supplementary information; they give the verb types in (16) as typical examples of non-main verbs. In order to fully appreciate what follows, it is necessary to point out that we used the term verbal complex in the quotation above as a translation of the Dutch notion werkwoordelijk gezegde from traditional grammar, which cannot readily be translated in English.

Haeseryn et al. (1997:47) try to use the essentially semantic characterization of main and non-main verbs to provide an operational definition in syntactic terms. The crucial criterion they mention is that any verbal complex contains exactly one main verb. When we apply this criterion to a perfect tense or passive example, this characterization goes in two ways: if we assume that the participles in (17) are main verbs, we should conclude that the temporal/passive auxiliaries are non-main verbs; if we assume that temporal/passive auxiliaries are non-main verbs, we should conclude that the participles are main verbs.

The one-main-verb criterion implies that main verbs crucially differ from non-main verbs in that they may but do not need to combine with other verbs into a verbal complex, whereas non-main verbs must always be combined with some other verb. This seems to work fine in the case of the examples in (17): the verb aaien'to pet' may indeed occur as the verbal head of a clause, whereas the temporal and passive auxiliaries cannot (although one must keep in mind that hebben can be used as a main verb meaning "to have/possess" and worden can also be used as a main verb if it is used as a copular expressing "to become", hence the number sign).

One may also welcome the one-main-verb criterion since it coincides nicely with our intuition that we are dealing with two predicational relationships in examples such as (19) and, hence, that it consists of two verbal complexes. That the verb horen'to hear' functions as a separate predicate can also be made visible by pronominalization of the italicized phrase in (19a); since horen is the only verb in (19b), it must be a main verb.

However, if we apply the same test to the examples in (20), we have to conclude that modal verbs like moeten'must' and mogen'may' are main verbs as well. This means that we can only maintain the claim that modal verbs are non-main verbs if we claim that clauses with modal verbs are exceptions the general rule that non-main verbs must be combined with a main verb; see Klooster (2001:55) for discussion.

There are many difficulties with maintaining that modal verbs are non-main verbs. First, it means we should assume that whereas example (19a) contains two separate verbal complexes, example (20a) consists of just one single verbal complex. Since there is to our knowledge no syntactic evidence to support this, adopting this conclusion voids the notion of verbal complex from any empirical content. For example, the embedded clauses in (21) show that the finite and non-finite verbs in (19a) and (20a) are part of a single verbal complex: the structure exhibits monoclausal behavior in the sense that the arguments of the infinitive must precede the finite verb in clause-final position (in the Northern varieties of Dutch).

For English it may perhaps be argued that modals like can are non-main verbs because they are like perfect auxiliaries in that they can precede negation and may undergo inversion with the subject in, e.g., questions (although this may also be a side effect of the accidental morphological property of modal verbs that they do not have an infinitival form, as is clear from *to can, as a result of which they are incompatible with do-support). See Quirk et al. (1979:120ff) and Huddleston & Pullum (2002:92ff.) for reviews of the criteria that are often used for distinguishing English auxiliaries, and Palmer (2001:100) for a more specific discussion of the English modal verbs.

In Dutch, however, there is no such syntactic evidence to indicate that the modal verbs in (16c) differ from that of run-of-the-mill main verbs; the only difference is that modal verbs have a defective present tense paradigm (there is no -t ending on the second and third person, singular forms). For this reason, we will not follow the characterization of the distinction of main and non-main verbs in Haeseryn et al. but simply assume that any verb must be considered a main verb that is predicative (has an argument structure) and can thus function as the head of a clause on its own; this reduces the set of non-main verbs by excluding, e.g., modal verbs like moeten'must'. See Section 4.6 for a more detailed discussion of the distinction between main and non-main verbs.