Have you been to Beijing before?
A: David, have you been to Beijing before?
A: 大卫，你以前来过北京吗？
B: Yes, I have. Many times.
B: 是的，来过。来过很多次。
A: Really? You must know it quite well.
A: 是吗？你一定非常了解北京。
B: Well, I think this is my sixth visit. I get to know it a little bit more each time.
B: 哦，我想这是第六次来北京了。我一次比一次更了解它。
A: Six tmies! That's a lot. It's nearly your home city!
A: 六次！不少啊。差不多成了你的家乡了！
Notes:
1. To ask someone if they have done something in the past, you can say: Have you been to Beijing before; Have you seen the Great Wall? 如果问某人在过去是否做过某事，你可以说：Have you been to Beijing before? (你以前去过北京吗？); Have you seen the Great Wall? (你见过长城吗？).
2. You must know it quite well. You can use this expression you must...when you think something for certain, e.g. after a long busy day: You must be tired. 你一定对它非常了解。当你非常确定某事时，你可以用下面这个表达法：You must...(你一定...), 例如：在某人忙碌了一天之后说：You must be tired (你一定累了)。