介词短语应该是sit down，表示:坐下来。with是sit down这个动作之后伴随着其他的动作。
例句:
He sat down with a most gloomy countenance by fanny .他在范妮身旁坐下，面色阴沉异常。She took a book at random and sat down with it open on her lap .她随意拿了本书，坐下把书打开放在膝盖上。
Finally she sat down with a sigh; this she could eat at her leisure .终于，她叹着气坐下来了，到了现在，她才可以悠闲地吃东西。
