Did you just start a sentence with "But"!?!? My 7th grade english teacher is not happy with you!
(I know there is nothing wrong with it, but some teachers really dislike it)
And don't even think about starting a sentence with 'and'.
I think we're taught rules like this to prevent us developing bad habits at the outset. Then, once we have more experience, we know when it's okay to break the rules.
One of my partners' favourite schooltime memories is of a teacher boldly proclaiming:
> "Me" can never be the first word of a sentence