watashi ga rusu ni shite iru aida ni, haha wa heya o soujishimashita.

While I was away from home, my mother cleaned the room.

Breakdown of watashi ga rusu ni shite iru aida ni, haha wa heya o soujishimashita.

• は: topic particle
• 私わたし: I
• を: direct object particle
• が: subject particle

Questions & Answers about watashi ga rusu ni shite iru aida ni, haha wa heya o soujishimashita.

Why is it 私が and not 私は at the beginning?

私が marks 私 as the grammatical subject of the subordinate clause 私が留守にしている間に (“while I am away”).

• In Japanese, inside a clause like X が … 間に, the subject is usually marked with が, not は.
• が answers “who is doing / in this state?” → “I am the one who is away.”
• は marks the topic, and you normally don’t introduce an extra topic inside this kind of time clause.

You could say 私は留守にしている間に in some contexts, but it sounds more like you’re re‑topicizing 私 and can feel a bit awkward here. The natural pattern for this time clause is 私が留守にしている間に.

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Why is it 母は and not 母が in the main clause?

Here 母は marks 母 as the topic of the main clause:

母は部屋を掃除しました。
As for my mother, (she) cleaned the room.

• は often introduces what the sentence is “about.”
• The subject of the action “cleaned the room” is already clear and un-contrasted, so は is more natural than が.

If you used 母が, it would sound more like you’re specifically identifying the doer, e.g.:

• 母が部屋を掃除しました。
  “(It was) my mother (who) cleaned the room.” (perhaps in contrast to someone else)

In this sentence, the neutral, default way is 母は.

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What does 留守にしている literally mean? Is there a verb 留守する?