When stating ‘I was about to’ you are informing someone that you were going to do something, but another event prevented you from doing it. It is similar to ‘I am about to’ but the difference is that you will not do it any longer. Since you will not do it any longer, it becomes a past event. That is why we use ‘was’ instead of ‘am’
“I was about to” 表示即将我要去做某事的时候，但是这时却发生了另外一件事阻止了我的计划。它的表达类似于“I am about to”，但是区别在于：用了过去式的“I was about to” 表示去做某件事的计划最后没有实施。
Here is an example conversionsation to help you understand.
我们通过下面的对话示例来理解：
A: Hi Mike. I need to ask you a favor. 嗨，麦克，我需要请你帮个忙。
B: Hey Bob. What do you need? 嘿，Bob，你需要什么？
A: Can you drive me to the convenient store? My car is in the shop and we really need to get milk.你能开车送我到便利店吗？我的车在店里，我们真的需要一些牛奶。
B: Sure. I can do that. 当然，我能帮你这个忙。
A: Were you in the middle of something? I don’t want to bother you. 你正在忙什么事吧？我不想打扰你。
B: I was about to watch a movie, but that can wait. 我正要去看个电影，但是不着急，我可以一会看。
In this case, he cannot say, “I am about to watch a movie” because he will not be watching it because he has to drive his friend. You use “I was about to” when you will not be doing something immediately because of some other event.
在这个例子中，不能说：I am about to watch a move, 因为他需要去开车送他的朋友，所以没能按原计划看电影。你用“I was about to ”来表达当你因为出现的突发状况而不能马上（按原计划）做某事的时候，
看下面的例句:
“I was about to go out.”
“I was about to go to dinner.”
“I was about to go to bed.”
“I was about to go to work.”
“I was about to say the same thing.”
“I was about to call you.”
“I was about to send you an email.”
“I was about to mow my grass.”
“I was about to order us some drinks.”
“I was about to watch television.”