Hello Yuan!

I will help you correct your notebook entry according to standard American English.

Playing by bike: 其实英文没有这种用法。可以修改成"Playing on Bikes"但是在英文里“玩/play”这个字是小孩子的活动。成人不"play"。可能应该说："Riding Bikes with my Friend".

日期: Saturday, March 17th，2012. Beijing. [OR: Saturday, 3/17/12. Beijing.]

I went out to ride bikes with my friend.

On this day, the weather in Beijing was very bad.

We did not want to go at first.

But we were so bored at home that we changed our mind at noon.

We decided to go to the National Stadium by bike.

But when we went there, we knew that we cannot ride bicycles in the square.

What a pity!

Then we rode back.

On our way back, we visited the China Science Museum.

But when we went out of the museum, it was raining.

It's lucky that the rain was not big. 其实英文没有“大下雨”这个说法；应该是"It's lucky that it didn't rain hard."

Keep up the good work!

Playing by bike

Saturday, 3,17th，2012 ,Beijing
I played with my friend on bicycles. That day, the weather in Beijing was very bad. We did not want to play at first . But We are so bored at home that we changed our mind at noon.We decided to go to the National Stadium by bike.
but when we went there, we learned that we can not ride the bicycle in the square.What a pity! Then we rode back.On our way back,we visited the China Science Museum. But when we left the museum, it was raining.It 's lucky that the rain was not heavy.:).

真的好！Please write more.