Talking in chat from other player's perspectives [duplicate]

I have been playing Minecraft 1.12.2 Java Edition on PC. One of my friends started to make me say stuff I wasn't saying in the chat. I don't know what command he was using but he basically made the chat say, "I am an idiot" from my perspective (which I never did) and I really want to know how to do it so I can make a comeback and troll him and other people the same way. Does anyone know how to do this?

he is for sure using a tellraw command. To use it properly advanced knowledge of MC-Commands and/or JSON is helpful, because the tellraw command requires it. I am working with a lot of tellraw commands, so i would recommend using online tellraw-editor. Google is your friend for that.

Use this to to replicate what your friend did:

/tellraw @a {"text":"< insertNamehere > I'm an idiot!"}

Extra Fake Player-Join-Message, maybe to get some laughs back :D

/tellraw @a {"color":"yellow","text":"Notch has joined the game"}

He probably used /tellraw (archive) and just imitated the name tag at the beginning of the line. You can only do that if you have permission to use commands.