YOU SAID:

once I was doing my homework but then I got the sudden urge to run up and down the staircase.

INTO JAPANESE

かつて宿題をしていたのに、突然階段を上り下りしたい衝動に駆られました。

BACK INTO ENGLISH

I used to do my homework, and suddenly I had the urge to go up and down the stairs.

INTO JAPANESE

宿題をやっていたのですが、急に階段を上り下りしたい衝動に駆られました。

BACK INTO ENGLISH

I was doing my homework, but suddenly I had the urge to go up and down the stairs.

INTO JAPANESE

宿題をしていたのですが、急に階段を上り下りしたい衝動に駆られました。

BACK INTO ENGLISH

I was doing my homework, but suddenly I had the urge to go up and down the stairs.