they were ate grasses.
已经有实意动词了不需要were,如果用were应该是过去进行时.
改为:they ate grasses(他们吃了草).
they were eating grasses(他们那时正在吃草).
i tried milked a cow
两个词同时做谓语是不行的.
milk应该做非谓语动词,应该用try to do something或try dong something结构.
改为:I tried to milk a cow(我设法给牛挤奶)
i tried milking a cow(我试着给牛挤奶)