ache is a continuous, dull pain that is often not very strong but lasts for a long time.
Common examples include:
1. headache - pain in the head
2. stomachache - pain in the stomach
3. toothache - pain in a tooth
4. heartache - emotional pain
The verb form means to suffer from a continuous dull pain.
ache这词儿啊，说的就是那种隐隐作痛的感觉，不是特别剧烈但是老在那儿磨叽，可烦人了。
常见用法有：
1. 头疼 - 脑袋瓜子嗡嗡的
2. 肚子疼 - 胃里翻江倒海的
3. 牙疼 - 牙神经一跳一跳的
4. 心痛 - 心里头憋得慌那种感觉
当动词用的时候，就是"疼得慌"的意思。比如说"My legs are aching after the marathon"，翻译过来就是"跑完马拉松我这腿啊，酸疼酸疼的"。
补充说明：这词儿跟"pain"不太一样，"pain"更强调突然的、剧烈的疼痛，而"ache"就是那种慢性的、隐隐约约的疼。咱们老话说的"酸疼"、"胀痛"、"隐隐作痛"都能用"ache"来表达。