When there is no comma before and, readers will scan the previous words to find some structural element to equate with what follows and/or to control it.

     Miss Patterson was separated by about six rows of chairs from
          the bad boys
             and
          their new recruits.

"Their new recruits" is equated in form with "the bad boys" and occupies the same position in the grammatical structure, for it is controlled (or "governed") by the preposition from just as "the bad boys" is. On the other hand, seeing a comma before and or or will prevent readers from expecting any grammatical connection between what follows the conjunction and what comes before it. The comma informs readers that the words after the conjunction contain all of the necessary parts of a sentence, and this knowledge makes interpretation easier.

     The bad boys sat in the last row with their new recruits,
             and
     Miss Patterson had to try to impose discipline from a distance.

Instructions: Drag and drop the parts in order to form a sentence. When you think your sentence is correct, click on "Check" to check your answer. If you get stuck, click on "Hint" to find out the next correct part.