A Handbook of the English Language

by R. G. Latham

[§ 505]. The third point to determine in the syntax of conjunctions is the certainty or uncertainty in the mind of the speaker as to the facts expressed by the propositions which they serve to connect.

1. Each proposition may contain a certain, definite, absolute fact—the day is clear because the sun shines. Here there is neither doubt nor contingency of either the day being clear, or of the sun shining.

Of two propositions one may be the condition of the other—the day will be clear if the sun shine. Here, although it is certain that if the sun shine the day will be clear, there is no certainty of the sun shining. Of the two propositions one only embodies a certain fact, and that is certain only conditionally.

Now an action, wherein there enters any notion of uncertainty, or indefinitude, and is at the same time connected with another action, is expressed, not by the indicative mood, but by the subjunctive. If the sun shine (not shines) the day will be clear.

Simple uncertainty will not constitute a subjunctive construction,—I am, perhaps, in the wrong.

Neither will simple connection.—I am wrong, because you are right.

But, the two combined constitute the construction in question,—if I be wrong, you are right.

Now, a conjunction that connects two certain propositions may be said to govern an indicative mood.

And a conjunction that connects an uncertain proposition with a certain one, may be said to govern a subjunctive mood.