Many languages have an expression like "as if" or "as though" for describing a fictitious comparison — something appears, or someone behaves, as if a certain thing were true, even though it is, in fact, not true at all. This is exactly what als ob and als wenn express in German — two fully synonymous connectors, with als ob somewhat more common in writing and formal speech. The key idea to grasp first is that these clauses always describe an outward impression or appearance, while the underlying reality is different or the exact opposite; Er tut, als ob er krank wäre means he is pretending to be sick while in reality he is not sick at all.
The grammar behind this structure rests on two linked rules. First, since als ob and als wenn introduce a subordinate clause, the conjugated verb is pushed to the end of that clause, exactly as with weil or dass. Second, and more importantly, the verb must be in Konjunktiv II, never the ordinary indicative, because this mood is what signals the assumption is unreal — without it, the sentence loses its essential meaning of distinguishing appearance from reality. When the impression concerns the present, Konjunktiv II Präsens is used (wäre, hätte, or other modal forms); when the impression concerns something that appears to have already happened, Konjunktiv II Perfekt is used — hätte or wäre plus Partizip II — as in Sie spricht, als ob sie ihn nie gekannt hätte.
There is also a very common shortened form: ob or wenn can be dropped, keeping only als, but this omission forces a change in word order — the conjugated verb returns immediately after als, in second position, as in an ordinary main clause: Er tut, als wäre er krank instead of Er tut, als ob er krank wäre. The meaning is identical; the difference is purely stylistic, between a longer, more formal version and a shorter, more conversational one.
The most frequent learner mistake is using the ordinary indicative after als ob, mistaking it for a plain descriptive comparison — this strips the sentence of its intended meaning entirely, since without Konjunktiv II nothing distinguishes it from a real comparison. A second common mistake is forgetting to invert the verb when using the shortened als alone, producing Er tut, als er krank wäre instead of the correct Er tut, als wäre er krank.