mud的形容词形式为什么不加l

Our horses plodded down the muddy track.

我们的马沿着泥泞小路蹒跚而行。

We waded waist-deep into the muddy water.

我们蹚进齐腰深的泥水里。

They left the car and struck out along the muddy track

他们下了车，然后开始沿着泥泞的'小路往前走。

It's difficult enough without muddying the issue with religion.

即使不把宗教牵扯进去，这个问题也已经够棘手的了。

The paper has turned a muddy colour.

报纸已变成土黄色了。