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.