Ache lasts longer but hurts less. Pain is more short term but ‘pains’ more!
So when you run 10 kilometers and come back, the sensation you feel in your legs is the ache while if you eat street food and catch a stomach infection you may feel pain in your stomach.
When you don’t sleep for two days straight, you get a headache but if you bang your head against a wall, you get a pain in the head.
Though both words gives negative vibes or can say unpleasant sensations that one feel in one’s body,but there is a slight difference while using them.
Ache is used when there is a continuous discomfort in a particular part of body.It is a part of pain.It is not as acute as pain.
However,pain is a broader term.It can be physical,emotional of mental discomfort which is difficult to ignore.It is usually stronger and more sudden.
For instance;I have a backache from last five years.
It means you have a continuous backache and is tolerable.
After hit by a car,I felt a sharp pain in my back.
It means a lot of intolerable short term pain in my back which can be cured by the medicines.