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 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.
[§ 615]. The use of the word that in expressions like I eat that I may live, &c., is a modification of the subjunctive construction, that is conveniently called potential. It denotes that one act is done for the sake of supplying the power or opportunity for the performance of another.
In English the word that, so used, cannot be said to govern a mood, although generally followed by either may or might.
It should rather be said to require a certain combination to follow it. The most important point connected with the powers of that is the so-called succession of tenses.
[§ 616]. The succession of tenses.—Whenever the conjunction that expresses intention, and consequently connects two verbs, the second of which takes place after the first, the verbs in question must be in the same tense.
I do this that I may gain by it.
I did this that I might gain by it.
In the Greek language this is expressed by a difference of mood; the subjunctive being the construction equivalent to may, the optative to might. The Latin idiom coincides with the English.
A little consideration will show that this rule is absolute. For a man to be doing one action (in present time) in order that some other action may follow it (in past time) is to reverse the order of cause and effect. To do anything in A.D. 1851, that something may result from it in 1850 is a contradiction; and so it is to say I do this that I might gain by it.