"Not to beat a dead horse" 點解?

It means "不要白費心機 " . Think about the horse is dead already. Why we still waste our time to beat it up? You can see more examples at the link below.

Hope it helps!!!

To beat or flog a dead horse. The idiom refers to a person who beats a horse to make it go even though it is dead, thus to doing something that is completely useless.
Example -
You are flogging a dead horse by asking him to lend you money - he hasn't even got enough for himself.

Not to beat a dead horse.
Means you are doing something with no avail.

Idom definition:

If someone is trying to convince people to do or feel something without any hope of succeeding, they are flogging a dead horse. This is used when someone is trying to raise an interest in an issue that no one supports anymore; beating a dead horse will not make it do anymore work.

or it could be explained: someone persists in doing something which is useless and has no impact at all.

Usage: You are flogging or beating a dead horse by asking her to come on time - she has never been punctual in her whole life.

In chinese translation: 白费尽心机

不摔打一個過時的問題