The R-word er behaves like a phonetically weak pronoun, whereas the other R-words behave like strong pronouns. This is illustrated in (115): spatial er behaves like the weak personal subject and object pronouns ze'she' and m'him' in that it cannot be used in coordinated structures; the other spatial R-words, on the other hand, can be used in this context, just like the strong personal pronouns zij'she' and hem'him'.

The assumption that er is a weak pro-form also accounts for the fact that, as a general rule, it cannot be placed in sentence-initial position; the (a)-examples in (116) show that the normal position of weak object pronouns is right-adjacent to the finite verb in second or the subject in non-topicalized position, and the (b)-, (c)- and (d)-examples show that the same thing holds for the locational pro-form er as well as prepositional and quantitative er.

Weak subject pronouns (with the exception of the weak third person masculine pronoun -ie'he') behave markedly differently in this respect and, as is shown in (117a), they may either occur in sentence-initial position or follow the finite verb in second position. Since it is generally assumed that expletive er acts as a place-holder of the subject position, it is not really surprising that it exhibits the same behavior as the weak subject pronouns.

Note in passing that the generalization that non-expletive er cannot occur in sentence-initial position does not hold if er is part of a larger constituent occupying the sentence-initial position. This is especially the case if the first constituent is a (nominalized) infinitival clause; in (118), we give examples involving the locational pro-form er as well as prepositional and quantitative er.

      Example (119a) shows that weak object pronouns are obligatorily moved into a position preceding clausal adverbs like waarschijnlijk'probably'. Example (119b) shows that the same thing holds for the prepositional er (unless one of the syntactic restrictions discussed in Section 5.3 prohibits movement of er), and (119c) provides a similar example with quantitative er. We do not give an example with locational er given that place adverbs can be base-generated in a position preceding the clausal adverbs.

Note that the strong form daar in (119b) may occupy a position within PP since the complete sequence daar op can be scrambled or be placed in sentence-initial position, as is shown in the primed (a)-examples in (120). The primed (b)-examples show that scrambling or topicalization is not possible with er op; this follows from the property of er illustrated in (119b) that it must be moved into some position external to the PP in the middle field of the clause.

      The discussion above has shown that the R-word er behaves like a weak pro-form that must be moved into the position indicated by means of +R in (121); the only exception is expletive er, which, like weak subject pronouns, may also occupy the first position in the sentence, indicated by XP. The other R-words are more like strong pronouns in that they need not move into +R, that is, they may remain in their original position, and they may also occupy the sentence-initial position XP.

      There are two further remarks that can be made with respect to the position of the R-words. First, it should be noted that what holds for the “pure” expletive form er in (122a) also holds for the occurrence of er in (122b), which functions simultaneously as expletive and as prepositional er. This shows that prepositional er is possible in clause-initial position, as long as it also performs the function of expletive. Note that example (122c) shows that quantitative er behaves differently in this respect: it can never precede the finite verb in second position. We will return to these cases in Section 5.5.3.

Second, it can be observed from the (b)- and (c)-examples in (123) that movement of weak R-words is clause-bounded, that is, the landing site must be in the same clause as its original position. The (a)-examples show that R-words again behave in a way similar to weak pronouns in this respect.