When does querer trigger the subjunctive?

Querer triggers the subjunctive when you want someone else to do something, because the two clauses then have different subjects: quiero que vengas (I want you to come). When you want to do the thing yourself - same subject for both - you do not use que or the subjunctive at all; you just use the infinitive: quiero venir (I want to come). Different subjects means que plus subjunctive; same subject means a plain infinitive.

What does ojalá mean and which mood follows it?

Ojalá means roughly 'I hope' or 'if only' and it always takes the subjunctive - there is no indicative option after it. Ojalá venga means 'I hope he comes'. The que is optional: ojalá que venga is equally correct. With the present subjunctive it expresses a hope about the present or future. The word comes from Arabic (law sha llah, 'if God wills'), which is a nice fossil of Spain's history.

Why is 'quiero que voy' wrong?

Two reasons in one. First, after querer que you must use the subjunctive, so voy (indicative) should be vaya. Second, and more fundamentally, if the subject is the same person on both sides you should not use que at all - you want quiero ir (I want to go). Quiero que voy fails on both counts: wrong mood and an unnecessary que.

The Subjunctive After Wishes

This is the gateway use of the subjunctive, and the one worth mastering first. The logic is clean: when you say you want, hope, wish or prefer that something happen, you are not stating a fact. You are projecting a desire onto a verb. Spanish marks that projection with the subjunctive.

This page assumes you can already build the forms. If vengas, sea, vaya and esté look unfamiliar, start with how to form the present subjunctive and come back.

The trigger verbs

These are the verbs of influence and desire. When followed by que and a new subject, they push the next verb into the subjunctive.

querer que - to want (someone) to: Quiero que vengas. (I want you to come.)
esperar que - to hope that: Espero que estés bien. (I hope you're well.)
desear que - to wish that: Deseo que tengas suerte. (I wish you luck.)
preferir que - to prefer that: Prefiero que lo hagas tú. (I'd rather you did it.)
necesitar que - to need (someone) to: Necesito que me ayudes. (I need you to help me.)
pedir que - to ask (someone) to: Te pido que vengas. (I'm asking you to come.)

The common thread: one person wants to bend reality so that another person does something. That gap between wish and reality is what the mood encodes.

The rule that catches everyone: different subjects

Here is the part English speakers get wrong more than any other. The subjunctive after these verbs only appears when the two clauses have different subjects.

Different subjects -> que + subjunctive:

Quiero que (tú) vengas. (I want you to come.) - I want, you come. Two people.
Espera que (nosotros) lleguemos pronto. (She hopes we arrive soon.)
Prefieren que (yo) conduzca. (They prefer that I drive.)

Same subject -> plain infinitive, no que, no subjunctive:

Quiero venir. (I want to come.) - I want, I come. One person.
Espero llegar pronto. (I hope to arrive soon.)
Prefiero conducir. (I'd rather drive.)

Notice there is no que in the same-subject versions. English uses "to + verb" for both ideas ("I want to come", "I want you to come"), which is precisely why learners over-produce que. Spanish refuses to put que there when the subject has not changed.

The mental test before you speak: is the same person doing both halves? If yes, infinitive. If the second half belongs to someone else, que plus subjunctive.

Ojalá

Ojalá is a standalone expression of hope, and it is the purest subjunctive trigger in the language because it has no indicative option at all. It means something between "I hope" and "if only".

Ojalá venga. (I hope he comes.)
Ojalá tengas razón. (I hope you're right.)
Ojalá no llueva mañana. (I hope it doesn't rain tomorrow.)

The que is optional after it - ojalá que venga is equally good. With the present subjunctive, ojalá looks forwards to a present or future hope. (Switch the verb to the imperfect subjunctive and it becomes a wish about something contrary to fact, but that is a later chapter.)

A small piece of history that helps it stick: ojalá descends from the Arabic law sha llah, "if God wills". Eight centuries of al-Andalus left it in everyday Spanish, and it still carries that flavour of hoping for something out of your hands.

Worked examples

Watching the subject decide the structure is the fastest way to feel the rule.

Quiero estudiar más. (I want to study more.) - same subject, infinitive.
Quiero que estudies más. (I want you to study more.) - different subjects, subjunctive.
Espera ganar. (He hopes to win.) - same subject, infinitive.
Espera que ganemos. (He hopes we win.) - different subjects, subjunctive.
Prefiero quedarme en casa. (I'd rather stay home.) - same subject, infinitive.
Prefiero que te quedes en casa. (I'd rather you stayed home.) - different subjects, subjunctive.
Ojalá sea verdad. (I hope it's true.) - ojalá always takes the subjunctive.
Necesito que me digas la verdad. (I need you to tell me the truth.) - different subjects, subjunctive.

Common mistakes English speakers make

Using the indicative after the trigger. The instinct is to say quiero que vienes, copying the indicative you already know. It has to be quiero que vengas. Once querer que with a new subject is on the table, the following verb is subjunctive. No exceptions in this construction.

Putting que + infinitive where Spanish wants either que + subjunctive or a bare infinitive. Learners produce quiero que venir, a hybrid of the two correct patterns. Spanish has no "que + infinitive" here. Pick one: same subject gives quiero venir; different subjects gives quiero que vengas.

Forcing que when the subject has not changed. "I want to leave" is quiero salir, never quiero que salgo or quiero que salir. English hides the rule because "to leave" looks the same whoever is leaving. Spanish does not, so always ask who is doing the second verb before you commit to que.

Master those three and you have the most useful slice of the subjunctive working correctly. The other triggers - emotion, doubt, denial - layer onto the same machinery.