The article considers the specificities of the infinitive as one of the non-finite forms of the verb in the Middle English period. The author shows that during the period under study the infinitives coincide formally, however, there is an almost complete discrepancy in the functions of infinitives. Evidence is given that in the Middle English period there did not use to be two forms of the infinitive, instead, there existed two different types of infinitives, inflectional and non-inflectional ones.