On the bank of the river,we found him ------ on a beach,with

第一个空 填sitting 是因为find sb doing sth.发现某人正在做什么
第二个空 填 fixed 是因为with sth done 表伴随状态 done 表被动
第二个可能比较难理解,给你几个例子
with+宾语+过去分词(短语)
He sat there with his eyes closed.他闭目坐在那儿.
All the afternoon he worked with the door locked.整个下午他都锁着门在房里工作
with+宾语+宾语补足语,构成with复合结构,宾语可以是名词或是代词,宾语补足语可以是介词短语、过去分词、现在分词、形容词、动词不定式、副词.且with后面的宾语与其后的宾语补足语有逻辑上的主谓关系
再问： with sth +宾补表示伴随的只能是done吗
再答： 不是，有很多种，这里面表示被动符合语意 形式一 with+宾语+形容词 He often sleeps with the windows open. 他常开着窗睡觉。 Don’t speak with your mouth full. 不要满嘴巴食物说话。 形式二 with+宾语+副词 He stood before his teacher with his head down. 他低着头站在老师面前。 He was lying on the bed with all his clothes on. 他和衣躺在床上。 形式三 with+宾语+名词 He died with his daughter yet a schoolgirl. 他去世时，女儿还是个小学生。 形式四 with+宾语+介词短语 She said good-bye with tears in her eyes. 她含着眼泪说了声再见。 He was asleep with his head on his arms. 他头枕着胳膊睡着了。 形式五 with+宾语+现在分词(短语) He fell asleep with the lamp burning. 他没熄灯就睡着了。 I won’t be able to go on holiday with my mother being ill. 因为妈妈有病，我无法去度假。 形式六 with+宾语+过去分词(短语) He sat there with his eyes closed. 他闭目坐在那儿。 All the afternoon he worked with the door locked. 整个下午他都锁着门在房里工作。 形式七 with+宾语+不定式(短语) I can’t go out with all these clothes to wash. 要洗这些衣服，我无法出去了。 With so many people to help us, we are sure to finish it in time. 有这么多人帮忙，我们一定能按时完成。
再问： 太给力了，你的回答已经完美的解决了我问题！
再问： 谢谢〜 理解了 含被动意思时就用done