When we were in my room before, a book fell off the shelf by itself, didn't it?
Do you remember seeing that book fall off the bookshelf when we were in my room?
ーWhen we were in my room before, a book fell off the shelf by itself, didn't it?
「前、俺の部屋にいた時、本が棚から勝手に落ちたよな？」
to fall off で「落ちる」
a book fell off the shelf by itself で「本が棚から勝手に落ちた」
ーDo you remember seeing that book fall off the bookshelf when we were in my room?
「俺の部屋で本棚から本が落ちたのを見たの覚えてる？」
Do you remember seeing ...? で「…を見たの覚えてる？」
bookshelf で「本棚」
ご参考まで！
The book fell off the shelf on its own, didn't it?
こんにちは。
様々な言い方ができると思いますが、例えば以下のような英語表現はいかがでしょうか：
The book fell off the shelf on its own, didn't it?
本が棚から勝手に落ちたよね？
didn't it? は「〜だよね？」のようなニュアンスで使うことができます。
ぜひ参考にしてください。
カナダ 
アメリカ合衆国 
