Think of the subjunctive as a different state of being, a hypothetical one. If I ask: Tienes un novio? I am asking about whether or not you have a boyfriend (technically, a fiancé, but I believe it's used in a more generalized sense these days), but I'm not proposing that it's just a hypothetical. If I say: No dudo que tienes un novio, then I am stating that I am sure you have a boyfriend - no doubts. If I say, though, that I doubt it: dudo que tengas un novio, then I am stating that the situation is hypothetical, not real. I could say: dudo que tengas un novio. Dime, por favor, tienes un novio? In this case, my uncertainty (whether or not you have a boyfriend) is stated first, using the subjunctive, and then the request for information follows in the indicative.

Now, for the record, there are languages when you can use a subjunctive in a question to indicate what you think the likelihood is of a positive answer - Ancient Greek is like that. In fact, it's one of the more interesting nuances that is often lost in translation. At the risk of being confusing, in Greek the present indicative shows you believe the question to have a true answer (e.g., when Satan tempts Christ in the New Testament, he uses this when he says "if you are the Son of God", which is interesting), while a past indicative shows you expect a negative response. A subjunctive indicates you think that the answer is probably true, and an optative shows that you're less certain. Be glad Spanish doesn't have an optative mood, by the way.

Anyway, back to Spanish:

You can also use the subjunctive for situations that you want to see happen. For example:

Viva el rey! - Long live the king! (lit. may the king live!). Obviously, if you were stating that the king is alive, it's just el rey vive.

Espero que tu digas la verdad! - I hope you're telling the truth! (I'm just hoping but I'm not certain, so the truthfulness is hypothetical).

Note, also, that usually the subjunctive in the present tense makes the verb appear to belong to a different conjugation - -ar verbs have the endings of -er verbs: ame, ames, ame, amemos, ameis, amen, while -er and -ir verbs have -ar endings: viva, vivas, viva, vivamos, vivais, vivan. Irregular verbs usually take the first person singular stem or something approaching it: tenga, venga, vaya, etc. Note also that the first person singular is identical to the third person singular in the subjunctive to make sure you don't have to figure out if, say, "amo" is indicative or subjunctive.