How do I form the subjunctive tense and when do I use it?

Usage

It is important to remember that, although in native Spanish the subjunctive tense is used in a huge range of different tenses (eg. pluperfect subjunctive, future subjunctive), for GCSE a grasp of the Present Subjunctive is more than adequate.

The first thing to understand is when it is necessary to use the subjunctive rather than the indicative.  There are a whole range of different situations but the most important thing to remember is that the subjunctive refers to a 'SUBJECTIVE' situation.

--> it is used to talk about wishes, desires, possibilities, opinions, recommendations and doubts.

--> on the contrary, the indicative refers to facts and statements that are considered to be certain/objective.

Considering the above differences, here are some common verbs that would trigger the subjunctive:

1) wishes --> desear, exigir, insistir, preferir, querer, pedir, necesitar;

2) emotions --> alegrarse, enojarse, sentir, encantar, lamentar, sorprender; 

3) doubt --> dudar, no creer, no estar seguro, no parecer, no pensar;

4) recommendations --> recomendar, sugerir, aconsejar, ordenar, mandar, insistir;

Formation

Take the 'yo' form of the present indicative and add these endings:

AR verbs --> -e, -es, -e, -emos, -éis, -en.

IR/ER verbs --> -a, -as, -a, -amos, -áis, -an.

eg. HABLAR: hable, hables, hable, hablemos, habléis, hablen.

Unfortunately, there are some irregular stems to learn on an individual basis:

caber -- quepo -- quep-

caer -- caigo -- caig-

conocer -- conozco -- conozc-

hacer -- hago -- hag-

lucir -- luzco -- luzc-

parecer -- parezco -- parezc-

poner -- pongo -- pong-

querer -- quiero -- quier-

tener -- tengo -- teng-

traer -- traigo -- traig-

valer -- valgo -- valg-

ver -- veo -- ve-