▲太阳落山不能用 set down。set down 的意义为“(使)坐下、放下、卸下、记下、制定、申诉”等。
▲根据《牛津英语搭配词典》，要表示“太阳落下；太阳落山；太阳西沉”，可用动词 set, sink, drop, dip 等，均不搭配副词 down。如：
It was getting cooler as the sun sank below the horizon. 随着太阳没入地平线，天变得凉快起来。
Even after the sun had set, the stones continued to give out heat. 太阳落山后，石头还是热烘烘的。
▲若要搭配 down 则用 go down。如：
The sun went down below the horizon. 太阳落山，消失于地平线下。
The temperature dropped as the sun went down. 太阳落山后，气温下降了。
Light diminished steadily as the sun went down. 随着太阳落山，光线越来越暗。