Usefuller, usefullest 還是 more useful, most useful？

每逢英文單詞後面是 ful / less / ble / some / ed / ing / ous 或多於 / 等於三個音節是，都是用 more...than / the most...。

例：正確：more useful
       錯誤：usefuller (-er)

       正確：more handsome
       錯誤：handsomer (-some)

       正確：more helpful
       錯誤：helpfuller (-ful)

       正確：more famous
       錯誤：famouser (-ous)

       正確：more beautiful
       錯誤：beautifuller (more than or equal to 3 syllables)
       

沒有usefuller，usefuliest，只useful and useless