The subjunctive is the mood of imagination or command. Apart from the verb to be, it has no form separate from the indicative, except in the third person singular of the present tense, where the subjunctive form is the same as the indicative plural (he have, not he has; he go, not he goes). Generally therefore, in sentences in which the subjunctive might be appropriate, neither the writer nor the reader need know or care whether the subjunctive is being used or not.
But the verb to be spoils this simple picture. The whole of the present tense is different, for the subjunctive mood is be throughout — I be, he be, we be, you be and they be. The singular (but not the plural) of the past tense is also different—I were and he were instead of I was and he was. In the subjunctive mood what looks like the past tense does not denote pastness ; it denotes a greater call on the imagination. Thus:
"If he is here" implies that it is as likely as not that he is.
"If he be here" is an archaic way of saying "if he is here".
"If he were here" implies that he is not.
The subjunctive is dying; the indicative is superseding it more and more. Its only remaining regular uses are:
(a) In certain stock phrases: "Be it so", "God bless you", "come what may", "if need be" and others.
(b) In legal or quasi-legal language: "I move that Mr. Smith be appointed Secretary".
(c) In conditional sentences where the hypothesis is not a fact: Were this true, it would be a serious matter.
If he were here I would tell him what I think of him.
(d) With as if and as though, if the hypothesis is not accepted as true, thus:
He spoke of his proposal as if it were a complete solution of the difficulty.
Other correct uses of the subjunctive may be found in contemporary writings, but it is probably true of all of them that the indicative would have been equally correct, and certainly true of many of them that the subjunctive has a formal, even pedantic, air. Jespersen says:
In the old language the subjunctive served in clauses to express various subjective moods, uncertainty, hesitation, diffidence, etc. But these feelings are no longer felt with the same force as formerly, and as the subjunctive is hardly ever used colloquially, it may now, to a great extent, be considered a literary trick to remove the style from everyday associations.