I was frightening that my brother was going to get stabbed.

英訳に間違いが有れば正してください。

彼が刺されるのではと怖かった。
I was frightening that my brother was going to get stabbed.

ヒント
形容詞

答え
frighteningをfrightenedにする
もしくはIをItにする

I was frightened that my brother was going to get stabbed.
It was frightening that my brother was going to get stabbed.

frighteningは人が主語にはならない。

人+be+frightened+that~
人が~を恐れる

It was frightening to hear that a neighbor was a murderer.
隣人が殺人者だと聞いてそれは恐ろしいことだった。