hold ctrl and it will dig down as level as it can according to the piece next to it. also holding ctrl and dropping dirt it will drop it as level as it can.
You know what let me do you another favor. If you have questions it is best to check a wiki before asking on the forums. More often than not you will get better information faster. The wiki's are not always correct but if you find an error help out and post it on the thread for the wiki you use. http://www.thesalemwiki.com/wiki/index.php?title=Digging http://www.salem-wiki.com/mediawiki/index.php?title=Digging http://salemwiki.info/index.php/Digging I think all of those are the exact same so whichever. If or when the wiki becomes hard to understand then the help section is a wonderfull place to start a topic.