This paper addresses the issue of complex structures in the framework of Role and Reference Grammar (Van Valin and LaPolla, 1997). These authors establish an explicit and clear-cut typology of complex constructions for English highlighting the role of operators (modality, aspect, tense, etc.). However, after analyzing a 1000-example corpus, several cases will turn out to be quite ambiguous and not so definite as regards clausal selection. Consequently, such examples will prove that the boundaries between complex structures as established by Van Valin and LaPolla (1997) cannot be considered as so limited and precise, and that they will usually have need of a more detailed account drawing evidence from other theoretical grounds