This essay formulates a rule for the use of the to+verb and verb+ing finite complements designed to help students and teachers of English as a foreign language. The rule results from an analysis of the distinction between the directional function and inceptive aspect of the to+verb form, rooted in the prepositional origins of to, and the imperfective aspect of the verb+ing form, rooted in its use in the continuous tenses. Drawing on the work of, among others, Steven Pinker, Ekkehard Kônig, and Thomas Egan, these contrasting functions are clarified and reinforced by other uses of the to+verb form, such as the to be going to future and the to+verb subject or subject complement, and the use of the verb+ing form as a gerund, action nominalization, and deverbal adjective and preposition. The result of this comparative and relational analysis is a simple rule by which language learners can understand and effectively use the two complements correctly.