Understanding the Subjunctive Mood

Understand that the subjunctive mood in Spanish is used to express various states of unreality such as doubt, possibility, necessity, and action that has not yet occurred. It is often associated with expressions of emotions, implications, and hypothetical situations.
Keep in mind that the subjunctive is often used after phrases that express personal reactions or judgements such as ‘me alegro de que’ (I’m happy that) or ‘es importante que’ (it’s important that).
Be aware that ‘esperar que’ (to hope that) and ‘querer que’ (to want that) also trigger the use of the subjunctive.
Note that the subjunctive is used after conjunctions of time and conjunctions of provision, fear and prevention. Also when two subjects in a sentence are different, you often use the subjunctive for the second verb.

Forming the Subjunctive

Learn to form the present subjunctive. Begin with the first person singular of the present indicative. Take off the ending, and add the appropriate subjunctive ending (-ar verbs: e, es, e, emos, éis, en; -er and -ir verbs: a, as, a, amos, áis, an).
Know that to form the subjunctive of regular -ar verbs, take the present tense yo form, drop the -o, and add the appropriate ending.
And similarly, for regular -er and -ir verbs, the endings are swapped; the yo form drops the -o and adds the appropriate subjunctive ending.

Remember there are many verbs with irregular subjunctives. Some common ones include: ser, ir, dar, saber, estar, and haber.
Be mindful that some verbs change stem in the subjunctive. For example, the verb ‘tener’ changes to ‘teng-‘ in all forms except nosotros and vosotros.

Understand that the future subjunctive is not generally used in modern Spanish. It is only occasionally encountered in legal documents or literature.

Finally, consistently practice conjugations as it is essential to master the usage of subjunctive mood in different contexts.