1他们想要坐飞机去上海
这句是 “通过某种方式去某地”的句型,以下几个都是对的:
(1) They want to go to Shanghai by plane.
(2) They want to go to Shanghai by air.
(3) They want to take the plane to Shanghai.
2他们想去上海坐飞机
这句是他们想要去上海,然后在上海乘飞机,去上海的目的是,在那里坐飞机去某个地方
所以,要先说 They want to go to Shanghai
目的是乘飞机:to take the plane (to + 动词原形 这个形式叫做 “动词不定式”,表示目的
所以,第二句,只能这样说:They want to go to Shanghai to take the plane.