Some stones were like a few animals sitting together
= Some stones were like a few animals that sit together
这里是现在分词做后置定语,animals 和 sit 是主谓关系.to sit 是不定式,不定式和现在分词做后置定语的区别就是不定式表将来,这里句子意思不表将来,故不用不定式.
We bought some souvenirs and beautiful clothes made of silk
=We bought some souvenirs and beautiful clothes that is made of silk
这里是过去分词做后置定语,修饰some souvenirs 和 beautiful clothes, clothes 和 made 是动宾关系,也就是被动关系.句子已经有谓语bought 了,你再加一个 are,就有两个谓语了.
望采纳,祝开心~~!