will be back,,why有個be字??

第一，will 係一個助動詞，一句句子之中，只有 will 唔夠，will 之後一定要有另一個動詞。同樣，would / shall / should / can / could / may / might / must 都必須有多一個動詞。
第二，back 呢度唔係動詞，解「向後／回去」時係 adverb。你覺得可以用 will back，係因為你用中文諗，「回來」係動詞。但英文講「回來」o既動詞，係 return。所以你可以講：
I will be back tomorrow.　或者
I will return tomorrow.

will 後面需要一個動詞
但back不是動詞
因此加be

Back is not a verb, but an adjective here. So you need a verb here.

I will back--- 呢句入面冇動詞﹔如果正常一句英文句子當中需有動詞才是完整。
I will be back -- 當中"be"就係動詞