Thou shalt not take the name of the Lord thy God in vain.
This has nothing to do with swearing, dammit. (I just threw that last in for effect.) Bad language is hardly ever mentioned in the rest of the Bible, so why would people think that God would make it #2 on the Big Chart and then forget about it? Even making promises with oaths -- the other swearing -- gets very little play, Old Testament or New.
False prophecy, now, that subject comes up a lot, with many variations played on the Carillon of Scripture (that metaphor didn't work out as well as I'd hoped). And that, my friends, is what is referred to as taking the Lord's name in vain is here. It means no forging God's signature under your own ideas. It means being very cautious and considered about making any claims that what you teach is The Gospel, or The Authentic Gospel.
Teaching is of course not only allowed, but encouraged. Check up on the penalties for false teaching before you start blathering, however. Liberal Christians rightly criticize conservatives for stepping way over the line in making claims about what God does and doesn't want in law and politics. Then they do the same thing themselves -- and worse, because they will often get together and put a denominational stamp on it. They just claim to speak for God in more elegant terms -- which is what God really wants, right?
Anyway, everyone just cut it out, y' hear me?