汉译英:当我到家时,我才意识到我把雨伞落在教室里了.

当我到家时,我才意识到我把雨伞落在教室里了.
1 Until I got home,I didn't realize that I had left my umbrella in the classroom
2 It was not until I got home that I realized that I had left my umbrella in the classroom
3 Not until I got home did I realize that I had left my umbrella in the classroom.
（英语中的三大句型都给你了.）

I didn't realize that I had left my umbrella in the classroom until I got back home.
或
Not until I got back home did I realize that I had left my umbrella in the classroom .
希望能帮到您。

When I got home, I realized I left the umbrella in the classroom.
I did not realize I left the umbrella in the classroom until I got home,