The most unproductive thought in life is: "if I knew then what I know now."

生活中最没有用的想法是：“如果我那时就知道就好了。”

youdao

The only thing I would change, if I knew then what I know now, is to make that warning louder, because back in 1997-98 nobody listened.

如果我以前知道现在所知道的东西，那么我要修改的唯一的东西是使警告更大声一点，因为在 1997 - 98 年，没有人会听进这些警告。

youdao

I'll get through to him [Brezhnev] somehow, and if I ever find out he knew about the situation and still let everything happen, then I know exactly what I'm going to do.

我会设法让他(勃烈日涅夫)知道，如果让我发现他明明了解所有情况但是还是让这一切发生的话，那我就清楚的知道我该干什么了。

youdao

If I knew then what I know now, I would have had to have included a discussion about air quality.

FORBES: Air Quality In China

"If I knew back then about the choices in family planning, I would've been able to control having children, " said Lopez, who has 12 children.

CNN: In Philippines, a 14-year fight for birth control

If I knew that advice then, you know Honors knew it.

FORBES: Honors Navy Video Scandal: Beyond The Naked Women Jokes