The guarantee could be worthless if the store goes out of business.

如果商店倒闭，这张保修单可能就没用了。

Training is worthless unless there is proof that it works.

除非能证明训练有效，否则就是白搭。

You feel you really are completely worthless and unlovable.

你觉得自己真的是一无是处、不讨人喜欢。