In some languages this interchange goes farther than in others; in the Greek, for instance, such is the case with εἰ, to a very great extent indeed.
Hence we must look to the meaning of the sentence in general, rather than to the particular conjunction used.
It is a philological fact that if may stand instead of since.
It is also a philological fact that when it does so it should be followed by the indicative mood.
This is written in the way of illustration. What applies to if applies to other conjunctions as well.
[§ 511]. As a point of practice, the following method of determining the amount of doubt expressed in a conditional proposition is useful:—
Insert, immediately after the conjunction, one of the two following phrases,—(1.) as is the case; (2.) as may or may not be the case. By ascertaining which of these two supplements expresses the meaning of the speaker, we ascertain the mood of the verb which follows.
When the first formula is the one required, there is no element of doubt, and the verb should be in the indicative mood. If (as is the case), he is gone, I must follow him.
When the second formula is the one required, there is an element of doubt, and the verb should be in the subjunctive mood. If (as may or may not be the case) he be gone, I must follow him.