翻译是：
We shouldn't drop litter casually when we take a bus.
shouldn't：不应该 drop litter：扔垃圾 casually：随意地 when：当...时候 take a bus：乘公交
英语翻译:当我们坐公交车时,我们不应该乱扔垃圾
翻译是：
We shouldn't drop litter casually when we take a bus.
shouldn't：不应该 drop litter：扔垃圾 casually：随意地 when：当...时候 take a bus：乘公交