You'd better be careful.

你要小心一点。

Get a load of this.

你听听这个。

If it ain't broke, don't fix it.

东西还没坏，就别急去修它。

They say it's going to rain tomorrow.

据说明天有雨。

Stop carrying the weight of the world on your shoulders.

别太操心了，没有你地球照样转。 （carry the weight of the world on one’s shoulders:自以为重任在肩。）

Wouldn't it be a good idea to pick up your room?

你是不是考虑把自己的屋字收拾一下。