In this dissertation I argue that English has a third type of if, that is, a declarative subordinator that introduces irrealis content clauses. I present evidence that strongly suggests that irrealis if-clauses function like VP-internal complements or subjects, and not like adjuncts. In syntactic tests like extraction, preposing, clefting, and constituent order, irrealis clauses behave predominantly like complements. Moreover, no other preposition with conditional or concessive meaning can be used to replace irrealis if, which strongly suggest that irrealis clauses should not be considered conditional protases. A close analysis of the semantics of irrealis clauses also points towards a non-conditional analysis. Irrealis clauses refer to hypothetical states of affairs, but no idea of condition is implied in their meaning. Irrealis clauses usually have to be extraposed, but some predicates like prefer, imagine and hate allow irrealis clauses without extraposition. This further supports my proposal that irrealis if should be considered a subordinator. Finally, I conclude by suggesting that when and how are also strong candidates for the subordinator label, and I put forward ideas for future research.