Learn Hardcore German: Wenn der Vorhang offen ist, sehe ich das Tal hinter dem Haus. - When the curtain is open, I see the valley behind the house.

Because wenn introduces a subordinate clause in German, and in subordinate clauses the finite verb always moves to the very end. You read subject (der Vorhang), then any complements (offen), and finally the verb (ist).

Why does the main clause start with sehe instead of ich?

After a comma closing a subordinate (Wenn…) clause, the main clause still follows the verb-second (V2) rule. The entire wenn-clause counts as the first position, so the finite verb (sehe) must come immediately after the comma. The subject (ich) then follows.

How do I know das Tal is in the accusative, and why does it look identical to the nominative?

das Tal is the direct object of sehe (“I see what?” – the valley), so it takes the accusative case. Tal is a neuter noun, and for neuter nouns the article das is the same in nominative and accusative, which is why you don’t see a change.

Why is it hinter dem Haus and not hinter das Haus?

hinter is a two-way (Wechsel-) preposition. When it indicates a static location (“where is it?”), you use the dative case: hinter dem Haus. If it expressed movement toward that position (“to behind the house”), you would use the accusative.

Why doesn’t offen have an adjective ending like offenes or offener?

Here offen is used predicatively after ist to describe the state of der Vorhang. Predicative adjectives in German remain uninflected. If you placed the adjective before the noun (attributive use), you would inflect it: der offene Vorhang.

When should I use wenn, als, and wann for “when” or “if”?

wenn is used for conditions (“if…”) and for repeated/general actions (“whenever”).
als is used for single events in the past (“when I was young…”).
wann is used in questions (“When are you coming?”).
In this sentence, wenn expresses a general present condition: If/whenever the curtain is open.

Do I always need a comma before a wenn clause?

Yes. German subordinate clauses introduced by conjunctions like wenn, weil, dass, etc., must be separated from the main clause with a comma.

Can I switch the order and say Ich sehe das Tal hinter dem Haus, wenn der Vorhang offen ist?

Absolutely. German allows moving the wenn-clause to the end. In that case, ich sehe remains verb-second (V2), and you still place a comma before the wenn clause.

Sometimes I see hinterm Haus instead of hinter dem Haus. Is that correct?

Yes, hinterm is a colloquial contraction of hinter dem. In formal writing you should use hinter dem Haus, but in speech and informal contexts you’ll often hear hinterm Haus.