Was having a helluva time getting a single line break while editing posts.  Found this post on a WordPress support forum…from poster “moshu“…thanks for info man.

“The OP is most likely using the stupid wysiwyg (ceterum censeo: it should be banned) and in that one hitting the Enter – starts a new paragpraph.

Use Shift+Enter for linebreak (br) and Enter – for paragraph (p).”