为什么这里用“needed”,而不是“need”?Eye-care professionals hold clinics for two weeks.Patients receive an eye exam and a used pair of glasses ,if needed.

为什么这里用“needed”,而不是“need”?Eye-care professionals hold clinics for two weeks.Patients receive an eye exam and a used pair of glasses ,if needed. it needed hard work to finish the job这里为什么用needed而不用need? it needed hard work to finish the job这里为什么用needed而不用need? just needed some time,to figure things outneed为什么在这里用过去式 It is still needed in China in the following years.中为什么用needed,为什么?needed是什么词性、形态 英语语法,help !help! help!All of us need to feel needed and admired.为什么feel后面是直接加needed而不是being needed或be needed,不是被动语态吗? 霍尔元件为什么用N型半导体 而不是金属 这个句子是什么时态Last october the famers needed help with the apple harvest there were many apples on the trees 为什么这里会用 were 了 the book that you bought yesterday is exactly/exact the one i needed.这里为什么用exactly?be 动词后不是要加形容词吗? 已知函数f（x）={n^2(当n为奇数时)；-n^2(当n为偶数时),且an=f(n)+f(n+1),则a1+a2+a3+...+a100=?为什么当n为奇数时an=a1+a3+a5+...+a99=n^2-(n+1)^2=-2n-1,而不是an=n^2+(n+1)^2an=f(n)+f(n+1) 这里用加的啊，怎么变成减 sorry ididnot know you --------them 是用need 还是 needed急为什么答案是needed? it's not needed to wear your shool uniform.中为什么要用needed 英语翻译这里为什么最后直接用一个yourself而不是用by yourself?一定要回答, 请教“What do you do at Christmas这里的介词为什么用at 而不是用on? I introduce myself to you.为什么这里要用myself而不是用me? Do you think to climb so high were difficult这里为什么用 were 而不是用 are The police are duty on the duty on road.这里为什么用are 而不是用is? the children have fun playing in water这里为什么用playing而不是to play?