How to get certain strings from a player chatting?

So, I am making this admin script where everything goes very well. My only problem is that I can't seem to choose which strings I want to read. I've used stuff like message:sub(4, 6) to get different characters, but when it comes to finding the player name entered I can't determine how long the player name will be. I need this information to get the strings coming behind the character name. An example is: /warn WaterFoox "REASON". I want to get the reason for why they got warned and put it into a frame that warns them. I've read on the wikis, but I don't really understand the explanation of whats listed here: https://developer.roblox.com/articles/Lua-Libraries/string#string.sub

Here are some of my code:

game.Players.PlayerAdded:Connect(function(plr)
    plr.Chatted:Connect(function(message)
        for i, admin in pairs(Admins) do
            if plr.Name == admin then
                message = string.lower(message)
                if message:sub(1, 6) == "/warn " then
                    for _,player in pairs(game:GetService("Players"):GetPlayers()) do
                          if message:sub(7) == player.Name:sub(1,message:sub(7):len()):lower() then
                        if player then
                            local TheWarn = Warn:Clone()
                            TheWarn.Frame.Frame.MSGTEXT.Message.Text = message:sub(8)
                            TheWarn.Parent = player.PlayerGui
                            TheWarn.Frame:TweenPosition(UDim2.new(0.34, 0,0.362, 0), "Out", "Quint", 3)
                            wait(10)
                            TheWarn.Frame:TweenPosition(UDim2.new(0.34, 0,1, 0), "Out", "Quint", 3)
                            print("Warned " .. player.Name)
                        end
                    end
                end
            end
        end
    end)
end)

I would appreciate an explanation of what I need to use and please comment anything I should change in my script. I'll be happy with everything I get!

Thanks in advance!

~WaterFoox