Variants in Current English Grammar by a F Rodionov

Modern English possesses numerous structures which are represented by two or more modifications having the same meaning and which can for this reason be used indifferently. These parallel modes of expression or variants form an integral part of English grammar. A knowledge of their use is essential to both student and teacher of English: to the former, because if a student knows only one mode of expression, he  naturally assumes any other to be ‘incorrect’ ; to the latter, because “somehow the idea has gotten abroad that, if there are two way saying a thing, one must always be better than the other.