这四句话里的as,奇数句里的as是介系词,表示作为,或看作,认作,当作,认为的意思,偶数句里的as可以作连接词,所以可以写成：
A takes English as a useful means for research work.
A considers the news as true.
A usually thinks of the stool as having four legs.
A usually thinks of the vase as broken.
希望我能帮助你解疑释惑.