We need to distinguish between finite and infinitival argument clauses. The examples in (16) show that the choice depends on the matrix verb: whereas propositional verbs like zeggen'to say' or beweren'to claim' can take either a finite or an infinitival clause, an irrealis verb like proberen'to try' is only compatible with an infinitival clause. Note that we assume that the infinitival clauses in the primed examples contain a phonetically empty pronominal element PRO that functions as the implied subject of the infinitival clause; we will discuss this element in more detail in Section 4.3.

Replacing finite interrogative clauses by infinitival ones does not seem to be always possible. Although example (17a') is acceptable, it belongs to a more formal register of the language—in speech we only find more or less fixed forms like Hij vroeg me wat te doen'he asked me what he was supposed to do'. Example (17b') is also acceptable, but the verb does not introduce a question; instead the embedded clause has the function of a request.

      That the verb determines the form of the clausal argument is also clear from the examples in (18), which show that while perception verbs like zien'to see' exhibit an alternation between finite and non-finite complement clauses, the causative/permission verb laten allows infinitival clauses only.

A comparison of the primed examples in (18) with those in (16) shows us that the verb also determines the type of infinitival clause; whereas the verbs in (16) take te-infinitivals, the verbs in (18) take bare infinitivals. Section 5.2 will provide a brief introduction to the different types of infinitival clauses.
      It seems that there are only few verbs that can be combined with a finite but not with an infinitival declarative object clause. Manner of speech verbs seem to prefer a finite clause as their complement, but judgments on the corresponding infinitival constructions appear to differ from case to case and from person to person. This is clear from a Google search (3/16/2012) on the string [Vfinite * te zullen] for various tense forms of the matrix verbs roepen'to call', schreeuwen'to yell' and huilen'to cry'. while there are countless examples in which these verbs are followed by a finite declarative clause, our search resulted in only a small number of cases in which they were followed by an infinitival clause. We found a relatively large number of examples such as (19a') with the verb roepen, but only two examples such as (19b') with the verb schreeuwen, and no examples such as (19c') with the verb huilen'to cry'. Nevertheless, the primed examples all seem passable for at least some of the speakers we have consulted.

At first sight, the (a)-examples in (20) seem to contradict the claim that there are few verbs that can be combined with a finite declarative clause only, but the (b)-examples show that we should be careful not to jump to conclusions.

The contrast between the two primed examples seems to be related to the preferred interpretation of the implied subject PRO of the infinitival clause. First, consider the primeless examples with a finite clause: the most prominent reading of (20a) is that Jan noticed or discovered something about someone else, that is, the subject pronoun hij'he' of the embedded clause is preferably interpreted as referring to some individual not mentioned in the sentence; example (20b), on the other hand, is also compatible with a reading in which Jan noticed or discovered something about himself, that is, in which the subject pronoun hij takes the subject of the matrix clause as its antecedent. The contrast between the primed examples can now be accounted for by referring to the fact that the implied subject PRO of the infinitival clause differs from the subject pronoun hij in that it must be interpreted as coreferential with the subject of the matrix clause; this makes the interpretation of (20a') as unusual as that of (20a) if the pronoun hij is taken to be coreferential with the subject of the matrix clause. See Section 4.3 for a brief introduction to the restrictions on the interpretation of the PRO-subject of infinitival argument clauses.
      It seems that, besides restrictions imposed by the interpretation of PRO, there are various other factors that may affect the acceptability of infinitival argument clauses. The examples in (21), for instance, suggest that the verb voorkomen'to prevent' can only select finite clauses; the pronoun hij in the (a)-examples can without difficulty be interpreted as coreferential with the subject of the main clause but nevertheless the primed examples are severely degraded.

The examples in (22) show, however, that the primed examples improve a great deal when we add an adverbial phrase indicating causation or manner.

The primed examples in (23), which are adaptations of actually occurring sentences on the internet, further show that examples like these become even better if the embedded clause is a passive or copular construction. Observe that the relevant issue is not that subjects of passive and copular constructions are internal arguments because the same thing holds for the subjects of unaccusative verb like vallen'to fall' in the (b)-examples above.

Although there are more potential counterexamples to the claim that there are only few verbs that can be combined with a finite but not with an infinitival declarative clause, we will conclude by pointing out the contrast between the two examples in (24). The reason why (24a) does not have an infinitival counterpart might simply be that we are dealing with an idiomatic expression (which is also listed as such in dictionaries); the options for substituting the finite clause in (24a) are very limited.

The examples in (25) suggest that there are also few verbs that can be combined with a finite but not with an infinitival prepositional object clause: the (a)-examples show that verbs such as verwachten'to expect' that normally take finite PO-clauses also allow infinitival complements; verbs such as vragen'to request' that normally take infinitival PO-clauses give rise to a degraded result with finite complements.

To our knowledge the factors affecting the acceptability of infinitival argument clauses have not been studied in detail but we provisionally conclude on the basis of the discussion above that verbs selecting a finite declarative argument clause may also take an infinitival clause in the normal course of things, but not necessarily vice versa. Future research must show whether this conclusion is tenable.