Indirect questions are formed in German with a complex clause formed by a main clause and a subordinate clause
This is understood better with an example:
A direct question would be:
Wann ist er geboren?
When was he born?
The same question in an indirect form would be:
Wir wissen nicht, wann er geboren ist
We don’t know when he was born
“wann er geboren ist” is the subordinate clause. Remember that subordinate clauses have very special rules: Verb at the end, with a comma separating the main clause.
As we already had explained in the article interrogation in German, there are 2 types of interrogative clauses in German:
Closed indirect questions are formed with the conjunction ob:
Sie hat mich gefragt, ob er ledig ist
She asked me if he’s single
Open indirect questions can be formed with any interrogative particle:
Kannst du mir erklären, wo du gestern warst?
Could you explain to me where you were yesterday?
As an example, we’ll show you some structures commonly used in indirect clauses:
Er hat mich gefragt, ob/wann/...
He asked me if/whether/when/...
Kannst du mir sagen, ob/wann/...?
Can you tell me if/whether/when...?
Ich bin nicht sicher, ob/wann/...
I’m not sure if/whether/when...