This is the 7700th most frequent Japanese word.
危なかっ
"危なかっ" is the stem of "危なかった," meaning "was dangerous" or "was risky."
In this sentence, "危なかった" is used as the past tense of the adjective "危ない," meaning "dangerous," to describe the state of the bridge.
その橋は古くて危なかった。
That bridge was old and dangerous.
Here, "危なかった" refers to a past situation that caused alarm or danger during swimming.
海で泳ぐときに危なかったことを思い出します。
I remember the dangerous situation while swimming in the sea.
In this example, "危なかった" expresses that the speaker experienced danger in a specific past moment, conveying a sense of relief afterwards.