In the past, their were only single-decker buses.

过去只有单层公交车。

In the past, there were only single-decker buses. Nowadays, some buses are double-decker buses.

在过去，只有单层公车，而现在，很多都是双层的公交车。

They go to work by car, on green single-decker buses.

他们乘汽车去上班，在绿色single - decker公共汽车。