Conditions in Latin are of two kinds: Indicative Conditions and Subjunctive Conditions. The difference is exactly the difference in the two moods: The Indicative Condition makes a statement about fact; the Subjunctive Condition makes a statement about potential . If you translate Indicative Conditions literally, you will never be incorrect (although you may sound weird: "If you will have said this, I will be happy."). You will need rules to translated Subjunctive Conditions correctly. [You may skip ahead to the Subjunctive Conditions , if you wish.]
The Indicative Mood makes statements and, so, indicative conditions makes factual or logical statements about events in terms of what precedes or conditions a particular conclusion. "If this happen, that happens." The relationship of condition to consequence, or protasis to apodosis, may be merely factual or it may be logical. The Future Indicative condition creates a special situation: logically you cannot talk about the future as a matter of fact. Consequently, the force of a future condition is rhetorically colored with confidence, sarcasm, or even with the implication of a threat: "If you do that, your whole family's gonna die." ("Are you threatening me?" "No, sir, just stating the facts."). However, the view that this condition is a threat is an interpretation of its rhetoric, not of its syntax. Syntactically, the indicative states what the author takes to be or presents as a factual relationship. Translation of indicative conditions into English is almost always easy and direct; transation of factual or logical conditions into Latin requires that you think about the precise timing of the events, since it is often the case that the condition must precede the conclusion. When this happens, you must use the appropriate form of the perfect tense of the indicative (present perfect, pluperfect, or future perfect).
The Subjunctive Mood used in conditions is a special usage of the potential subjunctive. The potential subjunctive presents a state or an act, not as fact, but as existing in the realm of possibility. It may be a fact, or it may turn out that it was a fact. When we say that the event is "presented as existing in the realm of possibility" we are not talking about what the event actually is, but only about how it is presented. The subjunctive indicates only that the speaker has imagined an event, has thought about it from the point of view of what is possible or likely (or even certain, but nevertheless imagined).
The present subjunctive is used when an event is thought of in the realm of present possibility: that is, it is a present potential ... (it may never happen, but) it could happen.... indeed, it could be happening now, if you imagine it happening now, or it could happen tomorrow. This is called "open potential" and, under usual circumstances, an "open potential" refers to the future. As a result, the present subjunctive is used in conditions identified as "should-would conditions" or as "future less vivid conditions." "Should - Would" provides a handy translation that works under most circumstances. "Future Less Vivid" is a misnomer and should be avoided. See examples of should/would conditions.
The imperfect subjunctive is used when an event is no longer possible. That makes sense. The rest of this story, however, is the result of idiom in Latin. Among the possible events (hence, the potential subjunctive: it imagines an event as possible), we might want to distinguish those that were possible but are not happening now from those that were possible in the past but didn't happen then. In Classical Literary Latin, the imperfect subjunctive became identified with events that were possible (hence, the imperfect, even in the English idiom) but are not happening now: "If it were raining, we would be singing in the rain." This presents the speaker as one who imagines rain as a present possibility, but also imagines or implies that it is not factually the case (in the present). It is handy that English also uses the imperfect to express this kind of present unrealized possibility. We can distinguish the imperfect subjunctive from the present subjunctive by stating that the imperfect imagines events as what was possible but is unfulfilled as of the present moment. Hence, the common name "Present Contrary to Fact." This name is accurate for most Classical Latin usage.
The pluperfect subjunctive came to be use for the event that was possible but closed (unfulfilled) already in the past. "If it had been raining (but it wasn't), we would have stayed home (but we didn't)." Again, it is handy that the English condition uses a pluperfect in the same way that the Latin uses the pluperfect subjunctive. This kind of condition is accurately called the "Past Contrary to Fact" condition.
Most grammars and introductory texts divide the Latin Conditions into three categories. The view presented here is both simpler and more accurate. The beginning student, however, should use the method that works best for him or her. The point, after all, is to read with accuracy and enjoyment some Latin. But for those who like neat, clear, and abstract descriptions of language and syntax, the above description should help.