The Escape [Esc] key in the upper left hand corner of your keyboard.
Chat with our AI personalities
In single player you cannot open the chat box without mods. In multiplayer you press 'T'.
On a Minecraft server, chat is always on. If one of your friends can't chat, then it is a problem on their end.
If you have cheats turned on (which you do when you create your world), open the chat box with t and type /weather clear
Yes. Go to multiplayer settings > Chat= off.
The deafult key to open the chat is T.