A em dash is a separation in a sentence or an interruption in written dialogue. Think of it as a comma on steroids. A comma indicates a pause, but a em dash is a shout—a related idea coming to make a greater point.
“Uncle Earl did not use the brain he was given—if he was even given one—when he lit the charcoal while leaning over the grill.”
“Ruth was the only person I could trust, but she was indisposed—or was she?”
“I felt the scab on my forehead—the result of another stupid move on my part.”
“That crazy sister of hers—talk about a nut case—just walked in.”
A em dash also indicates an interruption in written dialogue:
“I don’t think she wants anything from you except—”
“I’ve taken care of that stupid Mary Thompson—”
Suddenly the door shot open, and a woman waving a gun stormed in.
Unlike the em dash, the hyphen does not separate; it joins, as when forming a combined adjective.
“A 350-pound gorilla escaped from the zoo and ended up climbing the Washington Monument.”
“Here’s the low-down on that obnoxious guy who just walked into the bar with a cute prairie dog on his shoulder.”
A hyphen is also used to form compound numbers between twenty-one to ninety-nine or fractions such as one-fifth.
“It took the zookeeper seventy-five pounds of bananas to get the gorilla to come down from the monument.
“The obnoxious man was told at least twenty-one times that he was not welcome in the bar—but his prairie dog was.”
Finally, a hyphen can also join a prefix to a word such as bi-polar, co-conspirator, and self-aware.
I always welcome comments as to whether my tips need correcting.
P.S. I taught middle-school for many years. When a student pointed out a mistake I made, I was overjoyed because it was a sure sign she was paying attention.
Kathleen Kaska, Marketing Director