The default editor in the MT 5.12 is really hard to use. Fortunately, There are many alternatives.
CCKEditor is a fantasity/free/opensource WYSIWYG html editor fo your.
CKEditor for Movable Type
There is a plugin for you to use CKEditor in MT. Home page is here
Filesystem install
wget https://github.com/downloads/usualoma/ckeditor-for-mt/CKEditor-1.074.tgz
tar xvf CKEditor-1.074.tgz
cd CKEditor-1.074
cp mt-static/plugins/CKEditor /path/to/mt/mt-static/plugins/
cp plugins/CKEditor /path/to/mt/plugins/
Run:
Open your website/cgi-bin/mt/mt.cgi in the browser. MT will prompt for install new plugin, follow the step which will automaticly install and enable CKEditor your.
Then start write new entry, CKEditor is here for you, enjoy it.