一個是ing，一個是被動式。
writing是主動，written是被動，比如I am writing a book.跟A book has been written by me.從語態出發，writing是進行時，表示正在進行，written是完成時，表示寫完了的。
用作形容詞 (adj.)
I had already issued a written farewell to the troops.
我已向部隊發表了書面的告別詞。
The requirement of written form can only be waived in writing.
書面形式的要求只能透過書面的形式來予以放棄
