不可能だとわかった

found out it was impossible

→ 不可能だ
phrase Rare #32,245
found out it was impossible Used when someone realizes or discovers that something cannot be done. The subject and the specific action are usually stated separately in the sentence.
JA: 一人でこの問題を解くのは不可能だとわかった。
EN: I found out that it was impossible to solve this problem alone.

彼は徒歩でそこへ行くのは不可能だとわかった

He found that it was impossible to get there on foot.

私は優勝するのは不可能だとわかった

I realized it was impossible for me to win.

その本を読破しようとした不可能だとわかった

I tried to read the entire book, but found it impossible.

彼女は彼の言うことを理解するのが不可能だとわかった

She found it impossible to understand what he was saying.

私は英語で彼を負かすのは不可能だとわかった

I found it impossible to beat him in English.