Comma's before 'but'?

[Brenda Creech]

I like the idea of a style sheet to use because I always second guess myself over commas!

[emidio125]

SO I just got my first review score and all my errors are - there should be a comma before 'but'.
Do American's use a comma before 'but' all the time, because that probably means i'm going to fail the grammar section every time. It's not something I would ever do having been taught that it's incorrect my entire life...Also does that not cause problems when reviewing work from authors in Ireland or the U.K?

If the but is put after a dependent clause, there's no need to include a comma. However, if appears after an independent clause, then it needs a comma.