1 . be bad at 不善于。如：

I’m bad at maths. 我数学学得很差。

He is bad at playing tennis. 他网球打得不好。

2 . be bad for 对……不好。如：

Too much salt can be bad for you. 吃太多的盐对你不好。

Smoking is bad for you . 抽烟对你有害。

3 . go from bad to worse 变得越来越糟，每况愈下。如：

His business is going from bad to worse. 他的生意每况愈下。

We’re hoping for an improvement but things have gone from bad to worse. 我们希望有所改善，但情况却变得越来越糟。