When using the word 'was', you are referring to something in a past tense, or something that happened before. Combining it with the word 'busy' you can express something that was occupying you in a past time.

当用'was'这个词的时候，表明你在说过去时态的事情或者之前发生的事情，和'busy'这个词一起使用，用来表示在过去一些事情占据了你的时间。

Here are some examples: 这里有一些例子： 

"I was busy thinking."

 我刚忙于思考。

"I was busy working."

 我刚忙于工作。

"I was busy cooking dinner."

 我刚忙于做晚餐。

"I was busy talking on the phone."

 我刚忙于打电话。

"I was busy cleaning the house."

 我刚忙于打扫房子。

"I was busy studying for my test."

 我刚忙于我的测试学习。

.

"I was busy thinking of ideas for our website."

 我刚忙于为我们的网站想好的点子。

"I was busy entertaining our neighbors."

 我刚忙于招待我的邻居。

"I was busy completing my housework."

 我刚忙于完成我的家务。

"I was busy learning new things."

 我刚忙于学习新的东西。

By changing 'was' to 'am' you change your message from past tense to present tense and refer to something you are doing 'now.'

通过把'was'变成'am'，你就把过去式转换成了现在时，说明你现在在做什么。
Here are some examples: 这里有一些例子： 

"I am busy working."

 我正忙于工作。

"I am busy cooking dinner."

 我正忙于做晚餐。

"I am busy studying for my test."

 我正忙于我的测试学习。

"I am busy completing housework."

 我正忙于完成我的家务。

"I am busy talking on the phone."

 我正忙于打电话。