Abstract This paper attempts to account for the changing distribution of lexical subjects in English infinitives within the framework of the Minimalist Program, paying special attention to the role of the infinitival morpheme and the change in the category and formal features of the infinitive marker to. First, it is argued that from Old English to the 16th century, when the infinitival morpheme was present, the external argument of bare infinitives could be realized either as a lexical DP or the infinitival morpheme; then, the loss of the infinitival morpheme led to the situation that bare infinitives obligatorily have lexical subjects, as we see in bare infinitive complements to causative verbs in Present-day English. Next, focusing change in the category and formal features of the infinitive marker to, the following development of to-infinitives is proposed: (i) in Old English, to as a preposition had an inherent Case feature licensing the infinitival morpheme, which in turn functioned as the external argument of to-infinitives; (ii) in Early Middle English, to came to have a structural Case feature with the optional EPP feature, giving rise to to-infinitives with lexical subjects that are licensed by to; (iii) in Late Middle English, to began to lose its structural Case feature and have the feature specification with the EPP feature only as a functional category, giving rise to to-infinitives with lexical subjects that are licensed by the matrix v, as in the case Present-day English; (iv) this specification became the only possibility after the loss of the infinitival morpheme in the 16th century, at least for the types of to-infinitives investigated here.