The subjunctive is not used very often in English, but is is very common and important in Spanish. Some command forms are actually the subjunctive, so the formation of the present subjunctive should not be too difficult.

Present Subjunctive

-ar verbs		-er and -ir
-e -es -e	-emos -éis -en	-a -as -a	-amos -áis -an

The past subjunctive is formed from the third person plural of the preterite. Remove the -on ending, and add these new endings:

Past Subjunctive

all verbs
-a -as -a	-amos -ais -an

An accent is added to the stem vowel as well in the first person plural form. Instead of hablaramos, it is habláramos; instead of comieramos, it is comiéramos, etc. Note that there is another way to form the past subjunctive (a different set of endings), but the endings given are used more often.

The present perfect subjunctive is formed with the present subjunctive of haber and the past participle of the main verb. Similarly, the past perfect subjunctive is formed with the past subjunctive of haber and the past participle of the main verb.

Many verbs are irregular in the present subjunctive mood:

dar - to give		decir - to say/tell		estar - to be
dé des dé	demos deis den	diga digas diga	digamos digáis digan	esté estés esté	estemos estéis estén
haber - to have		hacer - to do/make		ir - to go
haya hayas haya	hayamos hayáis hayan	haga hagas haga	hagamos hagáis hagan	vaya vayas vaya	vayamos vayáis vayan
poder - to be able to		poner - to put/place		querer - to want
pueda puedas pueda	podamos podáis puedan	ponga pongas ponga	pongamos pongáis pongan	quiera quieras quiera	queramos queráis quieran
saber - to know		salir - to go out		ser - to be
sepa sepas sepa	sepamos sepáis sepan	salga salgas salga	salgamos salgáis salgan	sea seas sea	seamos seáis sean
tener - to have		traer - to bring		venir - to come
tenga tengas tenga	tengamos tengáis tengan	traigo traigas traiga	traigamos traigáis traigan	venga vengas venga	vengamos vengáis vengan

Many verbs are irregular in the past subjunctive as well:

dar - to give		decir - to say/tell		estar - to be
diera dieras diera	diéramos dierais dieran	dijera dijeras dijera	dijéramos dijerais dijeran	estuviera estuvieras estuviera	estuviéramos estuvierais estuvieran
haber - to have		hacer - to do/make		ir - to go
hubiera hubieras hubiera	hubiéramos hubierais hubieran	hiciera hicieras hiciera	hiciéramos hicierais hicieran	fuera fueras fuera	fuéramos fuerais fueran
poder - to be able to		poner - to put/place		querer - to want
pudiera pudieras pudiera	pudiéramos pudierais pudieran	pusiera pusieras pusiera	pusiéramos pusierais pusieran	quisiera quisieras quisiera	quisiéramos quisierais quisieran
saber - to know		ser - to be		tener - to have
supiera supieras supiera	supiéramos supierais supieran	fuera fueras fuera	fuéramos fuerais fueran	tuviera tuvieras tuviera	tuviéramos tuvierais tuvieran
traer - to bring		venir - to come
trajera trajeras trajera	trajéramos trajerais trajeran	viniera vinieras viniera	viniéramos vinierais vinieran

The main uses of the subjunctive include:

1. After the verb querer when there is a change of subject (but use the infinitive if there is no change of subject)

2. When one person tells (decir) or asks (pedir) another person to do something.

3. After expressions of emotion, such as esperar (to hope), sentir (to be sorry), temer (to fear), alegarse (to be glad), when there is a change of subject.

4. After dudar (to doubt) and other verbs expressing uncertainty.

5. After most impersonal expressions, such as es posible (it's possible), es importante (it's important), es necesario (it's necessary) if there is a subject for the subordinate verb.

6. In adjective clauses is the antecedent is indefinite

7. After certain conjunctions, such as para que (in order that), sin que (without), and antes que (before)

8. After time conjunctions, such as cuando (when), en cuanto (as soon as), hasta que (until), when futurity is implied.

9. In contrary-to-fact conditions, the past subjunctive must be used in the if-clause (and the main clause is in a conditional tense)

Beside the present and past perfect tenses, there are also the preterite, future and conditional perfect tenses. All are conjugated with a form of haber and a past participle.

The preterite perfect is formed with the preterite of haber + past participle, and it has the same meaning as the past perfect. But this tense is normally only used after conjunctions of time, such as así que, luego que, tan pronto como (as soon as); cuando (when); después (de) que (after); and hasta que (until).

The future perfect is formed with the future of haber + past participle and is also used to express probability, referring to the present.

The conditional perfect is formed with the conditional of haber + past participle and is also used to express probability, referring to the past.