[kǔ tou]

go through all sorts of hardships
吃尽了苦头

give him a lesson
给他一点苦头吃

These are lessons drawn from our setbacks in the past.
这些是我们吃了苦头总结出来的经验。

If you turn a deaf ear to the masses' criticism, sooner or later you will have to pay for it.
听不进群众意见， 早晚要吃苦头的。

[kǔ tóu]

Water from this river has a slightly bitter taste.
这个河里的水带苦头儿。