This paper was written in July 1999, but it has evolved out of a series of earlier papers and conference presentations variously called ‘Con PRO' (1995), ‘The case of Icelandic PRO' (1997), ‘Functional control with and without structure sharing' (1997)and ‘PRO versus structure sharing in Icelandic infinitives' (1997). The ante-penultimate version has been published as Hudson, Richard: 1998, `Functional control with and without structure sharing', in A. Siewierska and J. J. Song (eds.) Case, Typology and Grammar, Benjamins, Amsterdam, pp. 151-69. The present version was published in Research in Language (University of Lodz), Volume 1, pp 7-33, 2003.
In some languages a predicative complement or adjunct agrees in case with the subject of its clause, which provides a reliable clue for identifying the subject of non-finite clauses. This is important in deciding whether cases of 'functional control' need a syntactic analysis in terms of PRO or of 'structure sharing' (including NP-movement). The paper looks in detail at Russian, Icelandic and Ancient Greek, and concludes that in all these languages the choice between PRO and structure sharing is determined primarily by whether the overt nominal is subject or object of the governing verb, rather than by whether or not the latter assigns it a theta-role. In all these languages, a subject almost always demands structure sharing, regardless of theta-role status, but an object usually allows PRO, though it may also allow sharing and this option may depend on whether or not it bears a theta-role.