grammar

可能在你心中會想，back也可以作動詞用，為甚麼要多此一舉加個”ｂｅ”；但要解釋這一點，我們要明白句中，”when she will back”的意思是”甚麼時候她會回來”，所以，在這句裏，”ｂａｃｋ”是解作”回來；回到原處”

若”ｂａｃｋ”用作動詞沒有這個意味：
用作動詞時，ｂａｃｋ的意思是：後退、使後退；支持、下賭注於（馬）、 裱褙;給...加襯裡〔(+with)和背書(支票等)

用作容詞時，它的意思是：往後面；在後面；過期的；

若要符合上述句子的意思，就只有它用作adverb副詞是，才有這個意味：往後面；●至先前或原來的位置或情況
e.g. Put the dictionary back on the shelf 把字典放回書架上
　　Call the boy back　把那個孩子叫回來
　　We shall be back before dark　我們將在天黑前回來

因著”ｂａｃｋ”在你所提的句子中是副詞，所以便要加ｂｅ

2006-11-21 17:34:38 補充：
若更準確說，ｂａｃｋ在這句中是Adverbial particle副詞接語

2006-11-21 17:36:13 補充：
adverbial particle ：ph.1. 狀語助詞(指動詞後的副詞,如:come back, break down中的back, down)

2006-11-21 17:36:57 補充：
可參考yahoo字典

To make it simple.....perhaps u can understand more from these examples :

Tom asked Pete, ` When will Mary be back ? `
I want to know when Mary will be back.
Mary will be back in 15 minutes.

The word ` back ` here is an adj.

you hv to put "BE" here or there would be no verb in the sentence
back is an ADVERB here

back actually is a preposition
back also can be adjective
in this sentence,"When she will be back",that means where will she go back.
Go back is a phasal verb. Go is verb.if we want to use "back" alone to describe the above meaning.we need to add "be" as a verb.

May be i am wrong,but i hope i can help u.
if i am wrong,plz send a letter to me

In this sentence, the word "BE" is being used as a verb.
the word "BACK" alone by itself doesn't have the meaning of 番黎.

Besides, there is one more grammar mistake....
I don't suppose you "to" know when she will be back.

it need to add up be because it tells when she's going to be back. another answer is because it don't make any sense if you just write she will back. It is not a complete sentense. you can find out by checking you dictionary!

2006-11-21 15:10:35 補充：
because you also need a verb to know what you are doing.

when she will be back
呢一句, 因為沒有動詞
will只係一個助動詞, 但不能代替動詞
be= is, am, are
有時候, 無法介定用邊個(is, am, are)時
就會用be代表
be代表[是]