[Wolly aka Paolo Valenti]

I need that the reply box stay always under the topic not at the end of the replies.

Could you tell me how to do that?

Thanx in advance and sorry for my english.

[Stephen Edgar]

You would need to customize your templates. To get started with this follow the steps here to make copies of your templates for use in your theme.

https://codex.bbpress.org/theme-compatibility/

Then start by taking a look at loop-replies.php