认识   to know , to be familiar with , to recognize

了解   to know , to understand

知道   to know

(1) eg: A:你知道习近平吗？Do you know xi jin ping?

           B：我知道。他是中国主席。I know。 He is the Chairman of China.

           A：你认识他吗？ Do you know him?(met him before)

           B：我不认识.I don’t know him.

(2) eg  A: 你认识那个人吗？Do you know that person?

           B：我认识，他是William.我前两天见过他。Yes, I know. He is William. I met him afew days ago.

           A:William做什么？你了解吗? What is his job? Do you know ?(know more information)

           B: 我不了解，我是最近才认识他的。I don’t know .I knew him only recently.

(3) eg: 我以前去过那个地方，去那个地方的路我认识。I’ve been to that place.I knew the way to that place.

           我上个星期见过他，我认识他，可是我不了解他，我不知道他喜欢什么。I met him last week,I knew him,but didn’t really know him.I didn’t know what he likes.

           我认识你已经两年了，我了解你，我知道你喜欢什么，喜欢吃什么，喜欢喝什么。I knew you for two years.I really knew you .I knew what you like..what do you like to eat                and drink?

So  Grammar : “认识”、“了解”、“知道” + person （or place)“

”知道“and ”了解“ to get the information through the  certain way ，but “知道”is basic and superficial. So “了解“to know more and deep

“认识”and“知道“ are the same (to know basic and superficial)  but  ” 认识“ is to know directly ,.”知道”is to know indirectly.

“了解“and “认识”   ”了解“ is to know more information than “认识” 。“认识” is not sure to really “了解“。