You ask wenn warst du in Berlin? and what comes back is not the date. Wenn cannot ask anything in German, so the person across the table has heard the opening of a condition that never arrives at its second half, and they answer the sentence they can make out rather than the one you meant. The mirror of it goes out in writing: als ich in Berlin war, ging ich immer in dieses Café, meant as every visit you ever made, understood as the one stretch when you were living there. The correction arrives, it is a single word long, and it does not explain itself.
Three words, three jobs, and only one of them overlaps with anything.
Wann asks. It asks directly (wann kommst du?) and it asks inside a longer sentence, where the question is still a question even though the sentence around it is a statement: ich weiß nicht, wann er kommt. Als introduces one occasion in the past, or one unbroken stretch of it, and nothing that happened more than once: als ich klein war, als ich sie kennenlernte, als der Film zu Ende war. Wenn takes everything that is left, which is the present, the future, every condition, and, this being the part the short rules leave out, a repeated occasion in the past. That is why immer wenn ich in Berlin war is correct German and immer als is not.
Where any of the three opens a subordinate clause, the conjugated verb goes to the very end of that clause: als ich klein war, wenn ich Zeit habe, ich weiß nicht, wann er kommt. A direct question is not a subordinate clause, so there the verb stays in second place and nothing moves: wann kommst du? When the subordinate clause comes first, it occupies the front of the sentence by itself, and the main clause then opens with its verb: wenn ich Zeit habe, rufe ich dich an.
Als ich klein war, wohnten wir in Hamburg.
Immer wenn ich in Berlin war, habe ich sie besucht.
Wenn ich Zeit habe, rufe ich dich an.
Wann fängt der Film an?
Ich weiß nicht, wann er kommt.
The last two figures are one word doing one job in two sentence shapes, and they settle wann for good: where something is being asked, the other two words are not candidates, and the question mark has no vote either way.
The edge that trips people up
The short rule you will have met is “past and once, so als”, and that sentence is true read forwards and false read backwards. Past and once is als. Past on its own is not, because past and repeated is wenn, and immer wenn ich in Berlin war is the sentence the reverse reading loses. The test only runs in one direction, so what you have to know before you choose is not the tense. It is whether it happened one time. A past frame does other work in the sentence, mind: inside one, seit keeps its meaning while the verb goes past with it, which is set out in the post on seit and vor.
Als ich in Berlin war, habe ich sie besucht.
Two facts about the words themselves, rather than about their jobs. Als is not only this conjunction: it marks a role (als Kind, als Lehrerin, ich arbeite als Übersetzer) and it follows a comparative (größer als, besser als nichts). Nothing is derived there, and there is nothing to work out. Those are the same three letters doing unrelated work, and you already handle that in English with a word like “since”.
Als Kind war ich oft krank, und mein Bruder war größer als ich.
The other fact is about wenn, and it is a thing German simply does not divide. Wenn is also “if”. Wenn er kommt, sage ich es ihm is both “when he comes, I will tell him” and “if he comes, I will tell him”, and German does not choose between them here. What settles it is the situation you are both already in, not the sentence. If you have to be unambiguous, the language gives you other words when you need the conditional reading only (falls er kommt, sollte er kommen), but the plain sentence carries both readings and nobody is bothered by that.
How to hear it
Two questions, in this order, and the order is what makes it work.
First: is anything being asked? Not is the sentence a question, but is something being asked inside it, because ich weiß nicht, wann and ich habe ihn gefragt, wann are statements with a question buried in them. If something is being asked, you want wann, and you want it whether or not there is a question mark at the end.
Second, and only if nothing is being asked: did it happen once, in the past? Then als. Everything else is wenn: what is happening, what will happen, what would happen, and what used to happen again and again.
The single occasion against the repeated one is a distinction Italian carries in its verbs rather than in its conjunctions, splitting the past into two tenses for it, which is what the post on era against è stato is about. German keeps its two past tenses out of this and makes you say it in the conjunction, one word, at the front of the clause, before any of the rest of the sentence exists.
