The seven coordinating conjunctions used as connecting words at the beginning of an independent clause are and, but, for, or, nor, so, and yet. When the second independent clause in a sentence begins with a coordinating conjunction, a comma is needed before the coordinating conjunction:
 
Juan studied for his math test in the café, but it was hard to concentrate because of the noise.
The committee approved the proposal fairly easily, yet I felt that several members didn't really understand what they were approving.
Although there was a lot of discussion surrounding the declining numbers, we decided to move onto other agenda items, for there was still the important issue of increased expenses to discuss.
The school has had too few chemistry classes in spring semesters, so the president increased the number.