Conversion is very common in modern English. The main condition for the formation of a new language unit through conversion is the change of the syntactic function of the word, which is observed by the change of meaning. It is sometimes necessary to change the basic syntactic environment that is needed for conversion. A word formed by the conversion takes all the inflectional forms of the part of the language to which it belongs. Conversion, as a means of forming verbs and nouns, has become one of the characteristic features of the English language since the XIII century. The conversion formed as a result of the collapse of the inflectional system at the beginning of the Central Asian period. From 1150, a number of derivative verbs formed by conversion from adjectives appeared, and from the 13th-15th centuries, verbs derived from adverbs and participles appeared. In the Middle English period, until about the 15th century, the formation of verbs from nouns and adjectives significantly decreased due to the emergence of a large number of nouns and verbs borrowed from French. The grammatical homonymy of many English verbs and nouns resulting from the loss of inflection was only one of the prerequisites for the emergence of conversion as a new way of word formation.