I already know all this! Now, you're just preaching to the choir.

这些我都知道！现在，你只是在" 白费口" 。

Still Matthew said nothing and Marilla had a sense of having wasted words and breath.

马修仍然一言不发，玛丽拉有种白费口觉。

Save your breath, Gaby. It's not gonna happen.

别白费口了 加布 不可能。

Save your breath. She’s already made up her mind.

【breath】别白费口了，她意已定。

Her good advice was wasted on the children.

【waste】她对孩子们规劝是白费口了。

You're just shouting into the wind.

你只是在白费口。

Don’t try to reason with Paul – you’re wasting your breath.

【waste】别跟保罗讲道理，你在白费口。

Sometimes a person fights a battle that has been decided or keeps arguing a question that has been settled. We say this is like " beating a dead horse."

有时，一个人可能一直就已经决定了战争而战斗，或者一直争论已经解决了问题。我们说，这简直是白费口(就好象beating a dead horse)。