Sentences (1) and (2) have traditionally been related by a process that is called Verb Phrase Deletion (VPD).
(1) If I wanted to collect bottles, I would collect bottles.
(2) If I wanted to collect bottles, I would.
The earliest analyses of this phenomenon suggested that (2) was derived from (1) by a syntactic deletion rule (hence the ‘deletion’ in the name of the process – cf. Ross (1969a)). Later (Jackendoff, 1972; Wasow, 1972; Fiengo, 1974; and Williams, 1977a, among others), it was suggested that a null anaphor was generated in the base following would in (2), and that the semantic component read this anaphor as meaning collect bottles, hence accounting for the synonymy of (1) and (2). A third possibility is that (2) is generated in the base with nothing following would, would itself serving as a proform for would collect bottles. And fourth, (2) could be derived from (1), leaving would as a proform, in a process resembling pronominalization more than deletion (perhaps ‘proverbalization’).