There are two different ways to form the Conditional Mood (or Konjunctiv II & I) in German). There is a fully conjugated form or Simple Conditional, and there is also a Compound Conditional form that uses werden as a helper verb.
When to use the German Conditional
When talking about some hypothetical or imaginary situations.
Das wäre besonders widerwärtig. → That would be especially repulsive.
To make requests sound more polite.
Könnten Sie mein Bier passieren? → Could you pass me my beer?
With “if-then” type statements.
Wenn es regnet, bleibe ich zu Hause. → If it rains, I am staying home.
Various other “conditional” type circumstances.
Hätte ich gewusst, hätte ich dir einen Kaffee gekauft. → Had I known, I would havebought you a coffee.
The most important verbs to remember for the conditional form are the Auxiliary verbs. Especially keep an eye on the conjugation for werden.
Auxiliary Verbs conjugated in the German Conditional Mood
You can see that these conjugations all add an umlaut to the Simple Past form of the verb. It is most important to remember the conjugation of werden, because for most verbs the conditional is formed using the conjugated form of werden and the infinitive.
For the remaining verbs, the conjugated form of the Conditional is less common, however you may still see it from time to time. Just remeber that it will be almost identical to the Simple Past tense except with an umlaut change.
There is a second type of conditional mood that is primarily only used in newspapers and writing, so it’s not critical to learn but it is worth mentioning. You use this form of the conditional to distance oneself from the action being spoken about.
It’s probably used most often with sein and haben.
Examples of sein and haben conjugated in Attribution (Conditional)