Main and non-main verbs play a different semantic role in the clause. The former function semantically as n-place predicates and are therefore typically the (semantic and syntactic) head of a clause; sentences that contain two main verbs are thus normally biclausal. The fact that the addition of a non-main verb to a clause such as (80a) does not affect the number of arguments that can be expressed is normally taken as evidence that non-main verbs are not predicates. Instead, they are assumed to add, e.g., temporal, aspectual or modal information to the meaning expressed by the main verb.

Let us therefore for the moment assume that non-main verbs must, but main verbs cannot combine with another main verb in a structure exhibiting monoclausal behavior, and that we can test this for infinitival constructions by assuming that mono- and biclausal structures systematically differ with respect to verb clustering and the infinitivus-pro-participio (IPP) effect in the way indicated in Table 2.

	monoclausal	biclausal
verb clustering	+	—
infinitivus-pro-participio	+	—

The examples in (81) illustrate the monoclausal properties of structures containing the semi-aspectual verb zitten. First, example (81a) shows that the semi-aspectual verb and the main verb lezen form a verb cluster, as a result of which the infinitival verb zitten is separated from its direct object het boek'the book'. Second, example (81b) shows that the IPP-effect is obligatory.

We should note, however, that verb clustering is somewhat obscured in the varieties of Dutch spoken in Belgium since these allow permeation of the verb cluster by various elements; for example, the order in (81a) marked by a percentage sign is acceptable in some of these varieties. Further, we should note that passive constructions are exempt from the IPP-effect; we will ignore this here but return to it in Section 6.2.2.
      The examples in (82) illustrate the biclausal properties of structures containing the main verb beweren'to claim': example (82a) shows that the object het boek'the book' of the verb lezen'to read' can intervene between beweren and lezen'to read' and (82b) shows that the IPP-effect does not arise.

      Now consider example (83a), in which the verb proberen'to try' semantically functions as a two-place predicate with an agentive subject and an infinitival direct object clause. That we are dealing with a regular direct object clause is clear from the fact illustrated in (83b) that the infinitival clause can be pronominalized or be replaced by a referential noun phrase.

Example (83a) also shows that the infinitival complement of proberen can be either an om + te-infinitival or a te-infinitival without the complementizer om. We will see shortly that these infinitival complements exhibit a somewhat different behavior, but, first, the examples in (84) show that the two types of infinitival clause may be placed after the verb proberen in clause-final position, and that proberen must occur as a past participle in the corresponding perfect-tense construction. This is fully consistent with the earlier claim that proberen is a main verb.

The examples in (85) show, however, that the te-infinitival without om is special in that it is also compatible with the IPP-effect, provided that the object of the infinitival verb lezen precedes proberen: the word order in (85b) is unacceptable.

This shows that proberen may also trigger monoclausal behavior, from which we may conclude that it does not always behave like a run-of-the mill main verb, but may be of a hybrid nature in the sense that it also exhibit properties of non-main verbs. The fact that proberen is not an isolated case and that there are more unsuspected main verbs which can enter a verbal complex and thus trigger monoclausal behavior strongly suggests that having this option is not a defining property of non-main verbs. This is confirmed by the fact that constructions with bare infinitivals always exhibit monoclausal behavior, irrespective of whether the selecting verb is a main or a non-main verb: this is illustrated in (86) for the aspectual verb gaan and the main verb horen'to hear'.

All of this implies that the hypothesis that main verbs differ from non-main verbs in that they cannot combine with another main verb in a structure that exhibits monoclausal behavior is refuted, and, consequently, that we have to look for other means to distinguish main from non-main verbs.

This subsection investigates two other syntactic properties that seem related to the predicational nature of main versus the non-predicational nature of non-main verbs. The predicational nature of main verbs like beweren'to claim' and proberen'to try' is clear from the fact that they do not require a projection of a main verb as their complement; the primed examples in (87), in which the italicized infinitival clauses of the primeless examples are pronominalized or replaced by a noun phrase, unambiguously show that we are dealing with two-place predicates, that is, regular transitive main verbs.

Non-main verbs like the aspectual verb gaan in the (a)-examples in (88), on the other hand, are clearly not predicational, as is clear from the fact that they normally do not allow pronominalization of the projection of the infinitival main verb: the verb gaan is not able to license the subject and the object pronoun, which clearly shows that it does not behave like a transitive verb. A potential problem is, however, that the (b)-examples show that modal verbs exhibit unexpected behavior in this respect; example (88b') shows that pronominalization is possible (see also Section 4.5, sub II, where the same point was made).

Another potential problem is that we wrongly expect that main verbs always allow pronominalization of their infinitival complement. Consider the (b)-examples in (89) with the causative/permissive verb laten. Example (89b) shows that laten adds an argument to those selected by the embedded main verb lezen in (89a), from which we may safely conclude that it is a two-place predicate that selects a nominal subject and an object clause. Example (89b') shows, however that laten does not allow pronominalization of the embedded infinitival clause. The (c)-examples are added to show that perception verbs such as zien'to see' do behave as expected by allowing pronominalization of the embedded clause.

We have seen that there are two ways to establish whether a verb that combines with an infinitival verb is propositional in nature. The easiest way is to investigate whether it is able to introduce an argument that is not licensed by the embedded main verb; if this is the case, the matrix verb clearly has an argument structure of its own. The second way is to investigate whether the projection of the infinitival verb can be pronominalized; if so, we may conclude that the pronoun must be semantically licensed and therefore functions as an argument of the verb. Table 3 provides the results of these tests for the verbs in (88) and (89).

verb type	additional argument	pronominalization	example
aspectual	—	—	(88a)
modal	—	+	(88b)
causative	+	—	(89a)
perception	+	+	(89b)

Assuming that the distinction between main and non-main verbs is really determined by the question as to whether the verb is predicational in nature, we have to conclude that of the four verb types discussed here, only the aspectual verbs can be considered non-main verbs. This implies that the dividing line between these two sets will be slightly different than normally assumed in more traditional grammars. For example, whereas modal verbs are normally considered non-main verbs, we are bound to conclude that they are main verbs; see Klooster (1984/1986).
      For completeness' sake, we conclude by noting that the pronominalization test must be applied with care; not all structures containing the pronoun dat/het can be used to show that the verb under investigation is predicational in nature. There appear to be two complications. First, the examples in (90) show that secondary predicates can also be pronominalized by the pronoun dat; the intended interpretation of the pronoun is indicated by means of coindexing. The acceptability of the second conjunct in these examples does not show that the copular verb zijn is a two-place predicate; as Section 2.2 has shown, it is simply a verb taking a predicative small-clause complement.

Second, the examples in (91) show that left dislocation constructions should also be set aside. The fact illustrated in (91a) that the pronoun dat can be used to refer to the left-dislocated participle phrase does not show that the perfect auxiliary hebben is a two-place predicate. In fact, if we took example (91a) as evidence for assuming that the perfect auxiliary hebben is two-place predicate, we would be forced to conclude on the basis of examples like (91b&c) that it can also be a three- or even a four-place predicate, a conclusion that is clearly untenable.

We normally use the term complement as equivalent with the term internal argument; it refers, e.g., to arguments of verbs that are assigned a thematic role like goal or theme. Given that Section 4.6 has argued that main and non-main verbs differ in that only the former are predicational in nature, and that the latter are not able to select any arguments, we could restrict the term verbal complement such that it only refers to verbal arguments of main verbs. Nevertheless, we will adopt a somewhat looser notion of verbal complements that also includes the verbal projections in the domain of non-main verbs. The main reason for doing so is that we have seen that non-main verbs impose certain morphosyntactic selection restrictions on the main verb: perfect auxiliaries, for example, must combine with past participles, aspectual verbs only combine with bare infinitivals, and semi-aspectual verbs normally combine with te-infinitivals. By stating that non-main verbs select the projection of the main verb as their complement, these selection restrictions can be accounted for.

By discussing verbal complements of main and non-main verbs in a single chapter, it will also become easier to compare the behavior of such verbal complements. That this is desirable is clear from the fact that Subsection II has shown that besides clear-cut cases of main and non-main verbs, there are also verbs that are of a more hybrid nature; we will see numerous other cases in Section 5.2.2 and 5.2.3.

This section discussed a number of properties of main and non-main verbs. Main verbs function semantically as n-place predicates and are therefore typically the (semantic and syntactic) head of some clause; if the sentence contains two main verbs, they are prototypically expressed in a biclausal structure. Non-main verbs, on the other hand, are not predicates but provide additional information to the meaning expressed by the main verb. As a result, non-main verbs must combine with a main clause in a verbal complex and thus trigger monoclausal behavior; they exhibit the two properties indicated in Table 4, repeated from Subsection I.

	monoclausal	biclausal
verb clustering	+	—
infinitivus-pro-participio	+	—

It is nevertheless not always easy to determine whether we are dealing with a main or a non-main verb, given that some verbs exhibit a somewhat hybrid behavior, subsection II was devoted to the question as to how we can distinguish main from non-main verb. We argued that it is not sufficient to show that a verb enters into a verbal complex with an infinitival main verb and then draw the conclusion that we are dealing with a non-main verbs, given that main verbs like proberen'to try' also have this property. Therefore we decided that we need to investigate the predicational nature of the verb in question: if addition of this verb results in the addition of an argument that is not licensed by the non-finite main verb, or if the projection of the non-finite main verb can be pronominalized, we are dealing with a main verb. This leads to a classification slightly different from what is normally assumed in descriptive grammars. We illustrated this for modal verbs like willen, which are normally classified a non-main verbs but must be considered to be main verbs according to our criterion. Section 5.2 will show that this also holds for a number of other verb types.