Ⅰ. know “知道”;“了解”。侧重直接性。如：
① I know her very well. 我很了解她。
② I don’t know his current address. 我不知道他目前的地址。
Ⅱ. know about “知道知道关于……的情况”。
① I happened to know about him. 我碰巧了解他的情况。
② I don’t know about that matter. 我不知道那件事。
Ⅲ. know of “知道有……”;“听说过……”，侧重间接性。
① I know of the town but I’ve never been there. 我知道有那么个镇子，但从未去过那里。
② I don’t know Mark, but I know of him. 我不认识马克，但我听说过他。