Ok thats an easy one. I thought you wanted the hard one.
BTW, a hack is comming that will allow some really cool e-mail notifications but I have not gotten it to work on the "test" forum yet so it wont make it here till it does work.
Ok back to the quetion at hand.
To "watch" a thread or a post that you have not posted to look at the bottom of the page just below the "new topic", "post reply", "email friend" buttons for a link that says "Watch this thread" Click on it and you should get e-mails when people post replys to it.
If you are already watching a thread the link will change to "stop watching this thread".