Speaking of conjunctions, the word "however" gets its own special place in the pantheon of Generally Accepted Yet Totally Baseless Grammar Rules. The thinking was this: You shouldn't start a sentence with the word "however" first because it's a conjunction (see previous page) and second because "however" has a few different meanings, which might muddy the waters of understanding.
To wit: If you say "However long I live, I'll never forget you," the meaning of however is either "to whatever extent" or "no matter how" [source: Fogarty]. If you say, "However, I will forget you if I meet someone else," you obviously are using "however" like a qualifier. (The word "but," for instance, is pretty interchangeable with "however.")
Somehow, these two ideas led people to believe that "however" was a messy, inaccurate way to start a sentence. In fact, you'll be fine if you just follow these easy rules: Add a comma if you're using it to mean "but" and leave the comma out for an expression of extent.