Pitman's Commercial Spanish Grammar (2nd ed.)

II. The Subjunctive Mood is used after Impersonal verbs, as

Es menester que lo haga: It is needful that he does it.
Es necesario que lo haga: It is necessary that he does it.

EXCEPTIONS --

1. When the Impersonal Verb expresses certainty, as --

Es cierto que lo hará: It is certain that he will do it.

2. When the dependent verb has not its own subject, both the following constructions are correct --

Es necesario hacerlo: It must be done
Es necesario que se haga: It must be done

III. The Subjunctive Mood is used after the following locations --

Antes que (before)
En caso que (in case)
De miedo que (lest)
Por más que (however much, although)
Sin que (without)
Para que (so that)
Á menos que (unless)
Salvo que (except that)
Con tal que (provided that)
Sea que (whether ... or)
Aunque (even if)

IV. The Subjunctive Mood is generally used after the following locations, when the action refers to future time, but the Indicative is used when the action refers to the past or present --

Después que (after)
Aunque (although)
De modo que (so that)
No obstante que (notwithstanding)
Cuandoquiera que (whenever)
Hasta que (until)
Luego que (as soon as)
Quienquiera que (whoever)
Comoquiera que (however)
Cualquiera que (whichever, whoever)
Dondequiera que (wherever)
El 1°, 2°, 3°, etc., que (the 1st 2nd 3rd, etc., that)
El único que (the only one that)
El solo que (the only one that)
El último que (the last that)
Nada que (nothng that)
Ninguo, nadie que (no one that)
El mejor (or any other superlative) que (the best etc., that)

E.g. --

Escribió de modo que su padre quedó contento: He wrote so that his father remained content.

Escriba V. de modo que su padre quede contento: Write so that your father may remain content.

Le pagué aunque no hizo su trabajo: I paid him, although he did not do his work.

No le pagaré aunque haga su trabajo: I shall not pay him although he may do his work.

The above rules have liberties and exceptions to be learnt by practice, generally turning on whether the action is intended to be alluded to as a fact or as a mere conception.

V. The Subjunctive Mood is used after cuando, así como, luego que and similar expressions when the action of the verb refers to the future, as --

Lo haré cuando tenga tiempo: I shall do it when I have time.

N.B. -- The Future Subjunctive is often used in this case.

VI. The Subjunctive Mood is used after si (conditional if) when the context requires the following verb in the past form, as --

Lo haría si tuviese (tuviera) dinero: I should do it if I had money.

N.B. -- If the context requires the verb in the present form, the Indicative Present must be used, as --

Lo hago si tengo tiempo: I do it if I have time.

Lo haré si tengo tiempo: I shall do it if I have time.

When, as in the latter example a future time is indicated, the Future Subjunctive may be used instead of the Pres. Indicative, as --

Lo haré si tuviere tiempo: I shall do it if (ever) I have time.

VII. After como the Subjunctive Mood is used idiomatically but not necessarily, as --

Como le vió le habló: As soon as he saw him he spoke to him.

Como le viese le habló: On seeing him he spoke to him.

VIII. After creer, pensar, opinar, and similar verbs, the following verb is generally in the Indicative; but after no creer, etc., the verb is in the Subjunctive mood. After creer, etc., used interrogatively, the verb may be Indicative or Subjunctive.