How can I add line breaks to a tweet using Twitter.com? I have tried pressing enter, but that sends the tweet.
Shift + Enter
At least on OSX in Chrome, Shift + Enter adds a line break.
For Windows: In edit box press (do not release ALT button while you are pressing numbers)
ALT + (in numpad) 255
after you release ALT button, it will create a white space. Copy that and use as much you need.