没有错。flying over the river是airplane的后置定语。
Tim is looking at the airplane flying over the river.对 Tim正在看飞机在河上飞过,此句强调正在飞过
Tim is looking at the airplane that/which is flying over the river. 此句强调在河上飞过的飞机.
两句强调的东西不同
楼上的都不地道,这句子里的“看”一般用“watch”。这句话很简单:Tim is wathcing the plane flying over the river.
没有错误。river
对的。river