Oversimplified Guide to
German Subjunctive Mood

The Subjunctive (German der Konjunktiv) mood is used to talk about hypothetical but unlikely situations, often with a condition, introduced by wenn = if, and a result, introduced by dann. It is characterized by past-like verb forms in non-past contexts. Let's consider some English examples:

If he were here right now, I'd tell him a thing or two.

Analysis

To talk about hypothetical events in the the past, we use "subjunctive" forms of have + past participle:

If I'd (had) known then what I know now, I'd (would) have done things very differently.

Besides speculation about hypothetical situations, we also use the subjunctive in English to make a question or suggestion more polite:

Could you help me a moment, please?
Would you care for some more coffee?

Forms

In German, the subjunctive is formed in one of two ways, depending on the verb:

1. One-piece construction, in modern spoken German restricted to haben, sein, werden, wissen, a few other frequent verbs, and the modal auxiliaries, consisting of

Past Stem + -e-ful endings, e.g.
sein: ich war -> ich wäre, du wärest, er wäre, wir wären, ihr wäret, sie/Sie wären
(Strong and irregular verbs usually show Umlaut if possible; other examples kommen > käme, bringen > brächte; the modal verbs have the vowel of the infinitive.)

ich wäre, hätte, möchte, müsste, würde, wüsste usw.
(but wollte, sollte without Umlaut)
wir wären, hätten, möchten, müssten, würden, wüssten
du wärest, hättest, möchtest, müsstest, würdest, wüsstest ihr wäret, hättet, möchtet, müsstet, würdet, wüsstet
er / sie / es wäre, hätte, möchte, müsste, würde, wüsste
(as in Past tense, no final -t!)
sie / Sie ('they / you') wären, hätten, möchten, müssten, würden, wüssten

2. Two-piece construction, used with most other verbs, consisting of

würde + infinitive, e.g. sprechen: ich spreche -> ich würde sprechen
(du würdest, er würde, wir würden, ihr würdet, sie/Sie würden)

Crucial difference between German and English

In English
the choice between the "One-Piece" and "Two-Piece" construction is based on the kind of clause the form appears in
1-piece condition, i.e. if-clause If I had enough money
2-piece result, i.e. then-clause I would be somewhere on spring break.
In German
the choice between the "One-Piece" and "Two-Piece" construction is virtually automatic once you choose the verb
1-piece short list of common verbs
(see above)
Sie wären mir bestimmt böse,
2-piece all other verbs wenn ich das sagen würde.

3. Past vs. non-past time.

Despite their "past-like" appearance, the forms in 1 and 2 refer to present or future time. To refer to past times in the subjunctive, use the subjunctive of the appropriate auxiliary haben or sein plus the past participle of the main verb.

Wenn ich es bloß gewusst hätte, wäre ich nicht gekommen.

Using the Subjunctive

1. Wishes that are unlikely to be fulfilled

Most verbs Wenn subject (bloß/nur 'only') ...infinitive + form of würde, (dann) form of würde -- subject etc. ...verb in infinitive
  Wenn er mir nur helfen würde,
If he only helped me
dann würde ich es bestimmt schaffen.
I would definitely manage.
One-piece verbs   Wenn  subject (bloß/nur 'only') ...subjunctive form, (see above) (dann)  subjunctive form --  subject etc.
  Wenn du es mir nur kaufen könntest,
If only you could buy it for me
dann wäre ich glücklich.
I would be happy.

hätte ich..

2. als ob

als (ob)

3. ellipsis

4. politeness

5. Indirekte Rede