Difference of pain and ache in meaning:
1- He has a deep pain in his heart since she abandoned him.
2- I'm into going for a check up, I feel a pain in my head.
3- I have headache
"pain" has more general meaning but "ache" only belongs to diseases.
Structural differences of pain and ache:
- we say "painful" but we don't say "acheful"
- we say "headache" but we don't say "headpain"