字典解释

“sidetrack”是动词，意思是“受到了......的干扰”。从它的英文释义来看，这种干扰一般主要会影响到迫切或重要的事情，在口语和写作中都可以使用这个词。

例句

我们经常说突然被某事打扰而忘记做某事或分神，这个时候就可以说Ｉgot sidetracked.
I am terribly sorry i didn’t send you the file right away. I got sidetracked and forgot it.

或

Sorry I got completely sidetracked and didn’t attend to your request immediately.

我们一般说的“讲跑题了”也可以用sidetrack来表达：
Sorry I got sidetracked and talked about my lousy(糟糕的) experience at the bar. Where was I?
这里的意思是：刚才我跑题了，本来不该讲在酒吧的经历，应该接着前面的话题讲。这句话里“Where was I”的意思是“我刚才讲到哪来着？”

再比如，一些主播在做直播的时候，有时候为了照顾听众的问题，讲着讲着就跑题了，这是就可以说：
Live streaming hosts(流媒体主持人) are prone to be sidetracked by questions from audience.

翻译练习

约翰本想找一个信封，但没想到读起了一些旧时的信件。
Johh wanted to find an envelope but he got sidetracked and read some old letters.

参考翻译
John was looking for an envelope and got sidetracked reading some old letters.