We sometimes create idioms by standardizing a word or phrase. “Pain” is a great example of this. The phrase “Math is a pain” means that you don’t like math, that it irritates you psychologically. It is a soft way to complain about something or someone. However, you cannot change the word. For example, painful doesn’t mean quite the same thing. If you mistake the two, people might take you to the hospital.
Using painful instead of pain makes the sentence more serious. Look at these two examples:
The dentist was painful – This means the dentist actually caused you physical pain.
The dentist is a pain – This means going to the dentist is irritating psychologically. Perhaps he has bad breathe or tells rotten jokes. Maybe he doesn’t have free parking. But it doesn’t mean that he actually hurt you.
We often add a little to the idiom by saying where the pain is:
G rated statement: He is a pain in the neck
PG rated statement: He is a pain in the butt
R rated statement: He is a pain in the ass
What gives you a pain? Can English be a pain? Maybe your roommate. Write five sentences about who or what gives you a pain.
eslizgr8 
Dear Richard, When I was married the very hour that my mother-in-law was to arrive at the L.A. Airport for a visit of three weeks, I developed a muscle spasm in
my neck that was excruciating. Helpful neighbors dumped me in a station wagon and raced me to the E.R. at the local hospital. They plunged in an injection of
something that soothed my spasming muscles. Now, would you call this idiom one
of the neck or the butt. A pain in the neck or a pair in the butt. I’ve always wondered.
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