。
I don't know when mother will be back.
Literal
Mother [subject-が] when returns [question-か] [topic-は] don't-know (polite).
The embedded question 'いつ戻るか' ('when she returns, question') is then topicalized with は — かは is the composite marker for an embedded question functioning as a contrastive topic. The whole embedded question is the thing you don't know about. A compact 'I don't know [wh-]' structure worth internalizing.