Lynn "was walking" her dog when I saw her in the park.之所以不用walked是因為saw感官動詞。那When I arrived at my aunt's home , she "was sweeping" her kitchen floor.可以改用swept嗎？可是The robbers "were counting" the money when the police arrived.為什麼不能直接用count就好？它又不是while,只是when.