[§ 504]. The second point in the syntax of conjunctions is the fact of their great convertibility. Most conjunctions have been developed out of some other part of speech.
The conjunction of comparison, than, is derived from the adverb of time, then: which is derived from the accusative singular of the demonstrative pronoun.
The conjunction, that, is derived also from a demonstrative pronoun.
The conjunction, therefore, is a demonstrative pronoun + a preposition.
The conjunction, because, is a substantive governed by a preposition.
One and the same word, in one and the same sentence, may be a conjunction or preposition, as the case may be.
All fled but John.—If this mean all fled except John, the word but is a preposition, the word John is an accusative case, and the proposition is single. If instead of John, we had a personal pronoun, we should say all fled but him.
All fled but John.—If this mean all fled but John did not fly, the word but is a conjunction, the word John is a nominative case, and the propositions are two in number. If, instead of John, we had a personal pronoun, we should say, all fled but he.
From the fact of the great convertibility of conjunctions it is often necessary to determine whether a word be a conjunction or not. If it be a conjunction, it cannot govern a case. If it govern a case it is no conjunction
but a preposition. A conjunction cannot govern a case, for the following reasons,—the word that follows it must be the subject of the second proposition, and as such, a nominative case.