§ 421. _True notion of the part of speech in use._--In _he is gone_, the word _gone_ must be considered as equivalent to _absent_; that is, as an adjective. Otherwise the expression is as incorrect as the expression _she is eloped_. Strong participles are adjectival oftener than weak ones: their form being common to many adjectives. _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. § 422. _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 languages. 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. § 423. _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. § 424. _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.