True notion of the original form.—In the phrase I must speak, the word speak is an infinitive. In the phrase I am forced to speak, the word speak is (in the present English) an infinitive also. In one case, however, it is preceded by to; whilst in the other, the particle to is absent. The reason for this lies in the original difference of form. Speak - to=the Anglo-Saxon sprécan, a simple infinitive; to speak, or speak + to=the Anglo-Saxon to sprécanne, an infinitive in the dative case.
[§ 483]. Convertibility.—In the English language, the greater part of the words may, as far as their form is concerned, be one part of speech as well as another. Thus the combinations s-a-n-th, or f-r-e-n-k, if they existed at all, might exist as either nouns or verbs, as either substantives or adjectives, as conjunctions, adverbs, or prepositions. This is not the case in the Greek language. There, if a word be a substantive, it will probably end in -s, if an infinitive verb, in -ein, &c. The bearings of this difference between languages like the English and languages like the Greek will soon appear.
At present, it is sufficient to say that a word,
originally one part of speech (e.g. a noun), may become another (e.g. a verb). This may be called the convertibility of words.
There is an etymological convertibility, and a syntactic convertibility; and although, in some cases, the line of demarcation is not easily drawn between them, the distinction is intelligible and convenient.
[§ 484]. Etymological convertibility.—The words then and than, now adverbs or conjunctions, were once cases: in other words, they have been converted from one part of speech to another. Or, they may even be said to be cases, at the present moment; although only in an historical point of view. For the practice of language, they are not only adverbs or conjunctions, but they are adverbs or conjunctions exclusively.
[§ 485]. Syntactic convertibility.—The combination to err, is at this moment an infinitive verb. Nevertheless it can be used as the equivalent to the substantive error.
To err is human=error is human. Now this is an instance of syntactic conversion. Of the two meanings, there is no doubt as to which is the primary one; which primary meaning is part and parcel of the language at this moment.
The infinitive, when used as a substantive, can be used in a singular form only.
To err=error; but we have no such form as to errs=errors. Nor is it wanted. The infinitive, in a substantival sense, always conveys a general statement, so that even when singular, it has a plural power; just as man is mortal=men are mortal.