Last night,after a supper,i went for a walk with 短文改错

Last night,after supper,,i went for a walk with
some of my classmates..We were happy to see some
foreign friends take a walk,too.I said "Good night"
to them and they answered"Good evening"with
a smile.I was wondering on my way back to home.
Why did they say "Good evening"instead of Good
night"when they meet people at the night?Iwent to
the teacher for helping.Then I understood I made a
mistake with everyday English.Now I know it is very
important of us to use everyday English correctly.

Last night, after supper,（没有a) I went for a walk with some of my classmates. We happened（过去式） to see some foreign friends taking（强调当时的动作） a walk. I said "Good night" to them and they answered "Good evening" with a smile. I was wondering on my way back （back是副词，不加to)home. Why did they say "Good evening" instead of（固定短语） "Good night" when they meet people at night?（at night/i