she is sitting under the tree. 她正坐在树下. 主语：she 谓语：is sitting 介词短语作状语：under the tree. She is sitting under the tree.此句话的意思是“她正坐在树下.” is sitting作谓语,说明动作状态；under the tree介词短语作状语,修饰动作is sitting,表示地点. 介词短语是可以做表语的, 如She is under the tree.她在树下.under the tree介词短语作表语,说明She的状态.
