While the Subjunctive Mood is named for its use in dependent (subjoined) clauses, those dependent uses are developments of the Independent Subjunctive discussed here.
Independent Uses of the Subjunctive. The subjunctive has three independent uses. For an execration of the so-called deliberative subjunctive, click here ... but come back. Click here to open a window that outlines the dependent uses of the subjunctive. With two windows, you may compare dependent and independent uses.
Jussive Subjunctive. The Jussive Subjunctive may be used just like the imperative to give direct commands; in other words, it is the direct expression of the duty or obligation that rests on someone. Unlike the Imperative, however, you may use it in any number or person. So, you could say "Let us do something." or "Let them die." You can also use it to give orders, like "Don't do that!" The rules for the jussive subjunctive when it is used to give orders are fairly idiomatic and need to be memorized, but you can see that this use brings a great deal of flexibility to Latin expression. A most useful resource offered by the jussive subjunctive is its use in the past. Now it does not make sense to give an order in past time; no one can obey it. However, it does make sense to want to distinguish a mere statement (indicative mood) about your duty in the past from a direct expression of that duty. After all, in the present we can say "Do it!" (direct expression of duty; an order) or "You should do it." (comment about duty; a statement). The jussive allows the Latin speaker to make the same kind of distinction about the past: hoc debuit facere = "he was duty-bound to do this" (statement about duty) from faceret hoc = "he was to do this" (direct expression of duty). As a kind of imperative, a verb in the jussive subjunctive usually comes first in the sentence. Consider the position of imperative verbs in English. This is a feature of our common language that goes back millenia. The jussive subjunctive may be used to ask a question. The jussive question makes a request about duty and you can tell it is a jussive question if the answer is a command: "What am I to do?" "Go away!" quid faciam? abi! = jussive question with imperative answer. The jussive subjunctive is also used in subordinate clauses. (With two windows, you may place the dependent uses beside the independent uses and compare if you like.)
Potential Subjunctive. The subjunctive may be used to express an action as existing in the realm of possibility. Just as the jussive subjunctive (above) may be thought of as a direct expression similar to debeo + infinitive, so the potential subjunctive may be thought of a direct expression similar to possum + infinitive. As a direct expression of potential, the subjunctive indicates not that the speaker has observed a fact, but that he or she has imagined a fact, has thought about things from the point of view of what is possible, likely, or even certain. As a mood, the subjunctive insists on the speakers engagement with the thing or event. As a result the verb form implies various relationships that in English we express with helping verbs: could, may, might, etc. The potential subjunctive is perhaps the most flexible of the uses. You may use if for a hypothetical: "What would a real man do?" = Quid vir faciat? It may even be used to express a kind of polite hesitation: Consider the English, "I would like the class to be quiet." What does that "would like" really mean? Doesn't the teacher really mean "I want the class to be quiet." Why do we soften such expressions? I think the answer is that we are trying to be polite; we don't want to be too direct or "in your face." Latin can do the same thing: velim studentes tacere = "I would like the class to be quiet." The potential subjunctive may be used to ask a question. The potential question makes a request regarding how the addressee feels about the potential or likelihood of an event: "What can I do?" "You could sit still." = quid faciam? potes tacere. = potential question with a potential subjunctive (or possum) in the answer. NOTE: only context tells you whether quid faciam is a potential question or a jussive question. The potential subjunctive is also used in subordinate clauses. (With two windows, you may place the dependent uses beside the independent uses and compare if you like.)
Optative Subjunctive. The third use of the independent subjunctive is to express an action as existing as part of the speakers wishes, prayers or fears. There really isn't any handy idiomatic translation of this into English, at least not in the way "should" and "let" can translate the Jussive Subjunctive or "could" and "might" can translate the Potential Subjunctive. Sometimes an optative subjunctive in a prayer sounds like a command: "Hallowed be thy name!" or "Let us succeed!" Sometimes we can use "may": "May he die a miserable death!" or even "would": "Would that this were over!" Often it is easiest to identify what is meant (a prayer) and then to arrive at a good translation that suits the circumstances, even to say "I pray that ...." As a direct expression of emotional desire, a verb in the optative subjunctive usually comes first in the sentence. In this way it is much like imperatives in Latin and English and like jussives in Latin. It is often marked by ut or utinam as well, and editors are usually kind enough to put an exclamation point at the end of a optative expression. The grammarians of Latin do not identify any category such as an optative question. Either it does not exist, or it has not been noticed (It could happen: after all, the Deliberative Subjunctive has been identified, and it does not exist.) If you can say, "What am I to do?" asking for a command, you should be able to say something like "What do I really pray will happen to him?" asking for a prayer (or a curse) as an answer, "May he die miserably!" The optative subjunctive is also used in subordinate clauses. (With two windows, you may place the dependent uses beside the independent uses and compare if you like.)