PAIN vs. ACHE

There's  a slight difference between the words PAIN and ACHE. Both are most commonly used as nouns which describe an unpleasant sensation or feeling of discomfort usually associated with illness. But are they actually same?

There are some slight differences between those two words.

• ACHE is a feeling of a rather long term discomfort; not sharp enough to call it PAIN. Since it's persistent you may try to ignore it or get used to it. Also, ACHE usually forms compound nouns with body parts, i.e. a headache, a toothache, a backache

e.g.
Weather changes always give me a headache.

• PAIN is often short-lived, sharp and difficult to ignore. It's also more localised. It does not form compound nouns with body parts but rather goes like this: a pain in the leg, a pain in the neck or a leg pain, a neck pain.

e.g.
After falling off the stairs I felt a massive pain in my ankle.

Now look at the difference:
When you cut your finger you feel pain.
In next few days you may have ache in the wounded finger.

Both words are also verbs. However TO PAIN is most commonly replaced with TO HURT.

e.g.
Whenever I spend few hours in front of a PC, my eyes ache terribly.
Your unjust remark pained me. (Your unjust remark hurt me.)