Punctuation

Punctuation matters. A lot. 

The question of "Where does the comma go?" is, to me, one of the most important in Standard English grammar. Why? Simply because a comma can alter the meaning of a sentence. For example, let's take the case of direct address. 

I helped my uncle, Jack, off his horse.
I helped my uncle jack off his horse.

The lack of the direct address comma has dire consequences, as you can see. Another related common comma problem is the debate over the Oxford comma, which is used when listing a set of things. For example, instead of writing "Me, myself and I", you write "Me, myself, and I." Personally, I was taught to use the Oxford comma since I was very young and I faithfully advocate its use. Below are some other examples that show the difference it can make, taken from this website.

“I’d like to thank my parents, Jesus and Oprah Winfrey.” (Besides suggesting that a child was born to Jesus and Oprah, the lack of a serial comma also implies that Jesus’ last name is Winfrey.) 

"A notorious gambler, Charlie Sheen owed money to his ex-wives, Billy Bob Thornton and Hugh Grant.”

“This award is dedicated to my good friends, Young Jeezy and God.”

“…highlights of his global tour include encounters with Nelson Mandela, an 800-year-old
 demigod and a dildo collector.” (Note: this is a real example from The Times of London.)