This paper is an attempt to reanalyze the established order within the category of parts of speech, in particular prepositions and adverbs, as the units from these categories are characterized by the highest level of functional transposition in the language. We argue that overlapping of the categories was caused by the inclination of the Old English grammarians to take over main grammatical rules and exceptions, class division, definitions etc. from the Latin language. At the same time, grammatical phenomena existing in Old English were ignored or fitted to the already adopted Latin definitions and rules. That led to confusions, misunderstandings and overlapping in the grammatical system of Old English. The paper addresses the processes of lexicalization and grammaticalization which, according to our hypothesis, are observed within the category of prepositions in Old English and predetermined subsequent functional transposition. The analysis explains the phenomenon of functional transposition of the units which are simultaneously ascribed to several categories, e.g. prepositions, adverbs, particles etc. The class of space and time prepositions has undergone lexical and grammatical transformations which were consequently institutionalized in the language and the units became subjects to functional-semantic and functional-grammatical transposition the results of which are observed in present-day English. The former process resulted in transposition of the meaning, sometimes metaphorical, and further functioning of these units as adverbs, particles etc.; whereas the latter led to purely grammatical usage of the units, for instance substitution of the category of cases in English, uprise of new grammatical prefixes, loss of rigidness in word order.