Long before sth. 是在sth.(一件过去或现在发生的事)的很久以前。 For example: Long before the ancestors of human beings began to exist, the Earth was dominated by dinosaurs. 在人类的祖先开始存在于世（这个时间点）的很久以前，地球的统治者是恐龙。 Before long　是“不久之后”的意思。 For example: I hope to take you to the altar ere(before) long. (From Pride and Prejudice, Mr. Collins said to Elizabeth)　我希望不久之后就能带你走向神坛（结婚）。