place something into the middle of every post

Hi there,

You'd need to use a plugin like this: https://wordpress.org/plugins/ad-inserter/

Hope it helps :)

I prefer do not use plugin. I use child theme. Could you tell me how modify the php file?

I'm not sure how that would work, unfortunately.

It's likely done by filtering the_content, but I'm betting it's a very complicated process. I would definitely suggest using the plugin for something like this.