To the extent that voice, mode, and tense are accomplished by the use of agglutinated particles or inflections, to that extent adverbs and verbs are undifferentiated.
To the extent that adverbs are found as incorporated particles in verbs, the two parts of speech are undifferentiated.
To the extent that prepositions are particles incorporated in the verb, prepositions and verbs are undifferentiated.
To the extent that prepositions are affixed to nouns, prepositions and nouns are undifferentiated.
In all these particulars it is seen that the Indian tongues belong to a very low type of organization. Various scholars have called attention to this feature by describing Indian languages as being holophrastic, polysynthetic, or synthetic. The term synthetic is perhaps the best, and may be used as synonymous with undifferentiated.
Indian tongues, therefore, may be said to be highly synthetic in that their parts of speech are imperfectly differentiated.
In these same particulars the English language is highly organized, as the parts of speech are highly differentiated. Yet the difference is one of degree, not of kind.
To the extent in the English language that inflection is used for qualification, as for person, number, and gender of the noun and pronoun, and for mode and tense in the verb, to that extent the parts of speech are undifferentiated. But we have seen that inflection is used for this purpose to a very slight extent.
There is yet in the English language one important differentiation which has been but partially accomplished. Verbs as usually considered are undifferentiated parts of speech; they are nouns and adjectives, one or both, and predicants. The predicant simple is a distinct part of speech. The English language has but one, the verb to be, and this is not always a pure predicant, for it sometimes contains within itself an adverbial element when it is conjugated for mode and tense, and a connective element when it is conjugated for agreement. With adjectives and nouns this verb is used as a predicant. In the passive voice also it is thus used, and the participles are nouns or adjectives. In what is sometimes called the progressive form of the active voice nouns and adjectives are differentiated in the participles, and the verb "to be" is used as a predicant. But in what is usually denominated the active voice of the verb, the English language has undifferentiated parts of speech. An examination of the history of the verb to be in the English language exhibits the fact that it is coming more and more to be used as the predicant; and what is usually called the common form of the active voice is coming more and more to be limited in its use to special significations.
The real active voice, indicative mode, present tense, first person, singular number, of the verb to eat, is am eating. The expression I eat, signifies I am accustomed to eat. So, if we consider the common form of the active voice throughout its entire conjugation, we discover that many of its forms are limited to special uses.