A conditional sentence links a hypothetical event (the condition) to its consequence — a structure every language has, but German enforces a strict three-way distinction in degree of reality that many languages express only loosely, through context or a single word choice. Grasping this three-way split is the core of this lesson.
The first degree is the real condition (reale Bedingung): something genuinely possible in the present or future, phrased with ordinary Indikativ in both clauses, as in Wenn es regnet, bleibe ich zu Hause — rain is a real possibility here. The second degree is the unreal condition in the present: an assumption that contradicts current reality, or a wish unlikely to come true, requiring Konjunktiv II in the conditional clause and usually würde plus infinitive in the result clause, as in Wenn ich reich wäre, würde ich helfen — "if I were rich," not a genuinely open possibility. The third degree is the unreal condition in the past: regret, or imagining what could have happened but did not, built with hätte or wäre plus Partizip II in both halves, as in Wenn ich Zeit gehabt hätte, wäre ich gekommen — an assumption about an event that is over and can no longer be changed.
Structurally, when a sentence opens with wenn, the verb in that subordinate clause moves to the very end, followed by a comma and then the main clause, whose conjugated verb comes immediately, with no subject stated first — the wenn-clause itself occupies the subject slot as a single grammatical unit. wenn can also be dropped entirely, replaced by inverting the verb and subject in the conditional clause itself, as in Wäre ich du, würde ich es tun — a more literary style common in elevated writing and journalism.
The most frequent mistake is confusing the second and third degrees: using würde plus infinitive to talk about a past event that is already over, when hätte or wäre plus Partizip II is required instead. A second common mistake is forgetting that sein and haben in unreal conditions strongly prefer their original Konjunktiv II forms, wäre and hätte, over würde sein or würde haben, which sound clumsy and unnatural even though würde is standard with most other verbs.