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-