How can I absolutely know the fix?

A file could not be downloaded; the file content is different from what was expected.
Name: netty-all-4.0.15.Final.jar
URL: https://libraries.minecraft.net/io/netty/netty-all/4.0.15.Final/netty-all-4.0.15.Final.jar
File name: netty-all-4.0.15.Final.jar
Path: C: \ Users \ micha \ AppData \ Roaming.minecraft \ libraries \ io \ netty \ netty-all \ 4.0.15.Final \ netty-all-4.0.15.Final.jar
Exists: File

how can i fix it when i try to start minecraft forge 1.8 (even without mods) then it appears that i need help

Did you try to download the file Manuel and insert it into the directory?

I didn't understand sorry

Ok understand it

Hadn't worked because 1.8 no longer starts at all

So I can click the start button but nothing happens