1 Nephi 1:18

Therefore, I would that ye should know that after the Lord had shewn so many marvellous things unto my father Lehi, yea, concerning the destruction of Jerusalem, behold he went forth among the people, and began to prophesy and to declare unto them concerning the things which he had both seen and heard.

So I want you to know that after the Lord had shown so many wonderful things to my father Lehi about the destruction of Jerusalem, he went out among the people. He began to prophesy and tell them about the things he had seen and heard.

You need to know this: After the Lord showed my father these amazing things about Jerusalem's destruction, he went out among the people and started prophesying, telling them everything he'd seen and heard.