But this may be expressed compendiously. The sun shines, and the moon shines, may be expressed by the sun and moon shine.
Nevertheless, however compendious may be the expression, there are always two propositions wherever there is one conjunction. A part of speech that merely combines two words is a preposition—the sun along with the moon shines.
It is highly important to remember that conjunctions connect propositions.
It is also highly important to remember that many double propositions may be expressed so compendiously as to look like one. When this takes place, and any question arises as to the construction, they must be exhibited in their fully expanded form; i.e., the second subject, the second predicate, and the second copula must be supplied. This can always be done from the first proposition,—he likes you better than me=he likes you better than he likes me. The compendious expression of the second proposition is the first point of note in the syntax of conjunctions.
[§ 607]. 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.