So what is the subjunctive? To put it simply, it is a verb tense which suggests a certain degree of uncertainty. In English, it's very rarely used. We have 'be it' but use it rarely. A common use of the subjunctive in English is 'If I were you'. This exists in Italian, 'se fossi in te'. 'Fossi' may strike those, like myself until recently, as a surprise. Why not 'Se ero in te'? Because Romance language like Italian like to keep this tense very much alive. Unfortunately for English speakers, it means another set of verb conjugations to learn if we don't wish to sound like a moron.
So when to use the subjunctive? I have already mentioned uncertainty, but what context infers uncertainty? For starters, phrases which begin like this probably call for the subjunctive:-
Credo che... (I believe that...)
Suppongo che... (I suppose that...)
Immagino che... ( i imagine that..)
E' Neccesario che... (it's neccesary that...)
Non vale la pena che... (it's not worth it that...)
Penso che... (I think that...)
Can you see that all of these are slightly dubious phrases? It can express hope, fear, possibility. Penso che sia buono. (I think that it's good). It is very easy to get it wrong and think it could be used in place of the conditional. It can never be used to convey the same meaning as the conditional. Penso che sarebbe buono. (I think that it would be good).
There are also some linking words which are never present unless attached to a subjunctive verb, such as malgrado, nonostante, sebbene (despite/although).
Starting with regularities, it always makes you (and to be honest, myself) feel more confident.
So as to retain knowledge and not just replace every acquired tense with a new one, I find it most useful to compare the different tenses continuously. The rule for imperative verbs for example, requires a slightly opposite approach:-
Whilst with the subjunctive -ARE becomes I, with the imperative ARE is also an exception to the rule, as the informal and formal forms of the verb mix up, i.e. LEI conjugations become TU and vice versa. So instead of the way we'd address a professor in a statement-> Parla lentamente prof! (you speak slowy=statement) we say Parli lentamente prof! (speak slowly! imperative!). In the opposite case, if making a statement to a friend->Parli lentamente. (you speak slowly) --->becomes> Parla lentamente! If using an IRE or ERE verb which is irregular when imperative, thankfully it does not do this and maintains the normal method of conjugation, though obviously with more intonation E.g. Rimani con me (you remain/stay with me). Rimani con me! (stay with me!).
The three endings are so much easier when they stay the same, though it would seem for many tenses they like to part ways, even when conjugated regularly. Take the future and conditional tenses. Whilst -ARE and -ERE take an E before each ending such as Prenderò (Prendere, future first person singular), Lascerò (Lasciare, future first personal singular) whilst here -IRE decides to be the stubborn non-conformist one: Partirò (Partire, future first person singular). In past perfect tense -ERE gets a turn to be oh so rebellious, Credere (ho creduto) whilst -ARE and -IRE verbs are usually predictable: Sono andato (Past perfect, andare) and Sono uscito (past perfect, uscire). Naughty endings! Conform!
Back to the subjunctive, we can see that for the first three singular forms of the -ARE verbs take an i at the end if regular. Now let's take a look at -ERE and -IRE.
Noi-Crediamo (at least NOI and VOI know the subjunctive law!)
Example-Pulire (keep in mind that this is one of those verbs, like finire and capire which takes on SC... It keeps it with the subjunctive but keeps to the subjunctive rule)
Right, I shall start on irregular subjunctive tomorrow!