PPs headed by voorbefore and naafter are the only temporal PPs that can be modified. As shown in (87) and (88), the modifier can be an adjectival phrase or an adverb, a nominal phrase, and, in the case of na, a prepositional phrase headed by binnenwithin. These modifiers all indicate the “distance” on the timeline between the position of the reference object and the position of the eventuality expressed by the remainder of the clause.

The adjectival modifiers are mainly langlong and kortshortly in (89a). The adverbs are the same that are used for modification of spatial adpositional phrases: these involve vlak/palshortly (89b). The adjective snelquickly and the adverb spoedigsoon are often used as modifiers of na-PPs, as shown in (89s).

The adjectival modifiers lang and kort are antonymous expressions, which is rather peculiar given that antonymous expressions cannot be used to modify spatial voor-PPs. The fact that lang and kort are antonyms explains the contrast found in (90a); these examples show that adjectival modifiers are gradable and can be extracted by wh-movement, but that, as always in the case of antonymous APs, the neutral form (here: hoe langhow long) is preferred to hoe korthow shortly; cf. Section A23.3.2.2, sub ID. Example (90b) is ungrammatical due to the previously established fact that adverbs are not gradable. Example (90c) shows that both snel and spoedig can be questioned; the question mark indicates that we consider hoe spoedig to be somewhat marked, which may account for the fact that it occurs much less frequently on the internet than hoe snel.

Note that, as in the case of spatial PPs, the modifier does not modify the preposition, but the whole PP. This is clear from the fact that an R-pronoun can intervene between the modifier and the preposition, as is shown in (91).

As discussed in Section 33.4.1, sub IIA, adverbial clauses preceded by sequences like voordatbefore or nadatafter can be analyzed either as clauses introduced by a complex subordinator or as PPs headed by the preposition voor/na with a finite clause as their complement. Whatever the correct analysis, the cases in (95) and (96) reveal that such temporal adverbial clauses can be modified in the same way as PPs taking a noun phrase as their complement; cf. (87) and (88). Since the modifiers behave in the same way with respect to questioning, we will not illustrate this here.

However, the examples in (97) show that infinitival temporal clauses differ from their finite counterparts in that they do not allow modification. Note that we cannot illustrate this for voorbefore, since it cannot be used to introduce infinitival clauses; the corresponding infinitival complementizer is alvorens; cf. Section 33.4.2.