A  : Did you hear what she just said  ?
B : No, I wasn't listening. I was thinking about someone else
I need some help on this conversation. These are my questions :
Why is the tense (B said) in the past ? How about's " am not listening/ am thinking" ? 
Maybe the action "think" is continuous because the girl has just said recently.
Thank you.
Hello, tmhung - and welcome to English Forums.

First: It is better to type your text directly into the message box rather than copy and paste from some wordprocessing software, which in your case makes your type too small to read when I am trying to answer it. Thank you.

Now: It is possible that speaker B could use present continupus ('am not listening/thinking') instead of past continuous ('wasn't listening/thinking'), but then it would have a different meaning. In the original, B means that his lapse in attention happened in the past (when the girl spoke), but he could very well be paying attention to her now. If B uses the present continuous, however, it means that he is still not paying attention to her now.
Big sorry for that inconvenience in the previous post. I have tried to edit it but I didn't see the "edit" button, would you please show me how edit a post ?

Your answer is simple and clear to me.Thank you so much.
Students: We have free audio pronunciation exercises.
You cannot edit after a response has been made (in fact, I am not sure that you can edit it before, either). That's why I encourage posters to check what they have written before they post.
Ok, I'll keep that in mind. Thanks a lot.