当时我正在读书。

I was reading at that time.

我正在读书。

I am reading a book.

我是张鹏。你正在做什么? 我正在洗碗。我正在读书。

This is Zhang Peng. What are you doing? ------ I'm doing the dishes. I'm reading a book.

状语从句，我正在读书就在这时门铃响了

I was reading when the bell rang.