The indicative mode would be properly used in the following sentence, when the statement is regarded as true: If that evidence is true, then he is a criminal. Similarly: If he is rich, he ought to be charitable. Most directly declarative statements are put in the indicative mode.
But when the sense of the statement shows uncertainty in the speaker's mind, or shows that the condition stated is regarded as contrary to fact or as untrue, the subjunctive is used. Note the two sentences following, in which the conditions are properly in the subjunctive: If those statements be true, then all statements are true, Were I rich, I might be charitable.
The subjunctive is usually preceded by the conjunctions, if, though, lest, although, or the verb precedes the subject. But it must be borne in mind that these do not always indicate the subjunctive mode. The use of the subjunctive depends on whether the condition is regarded as a fact or as contrary to fact, certain or uncertain.
It should be added that the subjunctive is perhaps going out of use; some of the best writers no longer use its forms. This passing of the subjunctive is to be regretted and to be discouraged, since its forms give opportunity for many fine shades of meaning.