We haven’t discussed yet ______ we are going to place our new furniture.为什么不能用that ,要用where.顺便问一下宾语从句中如果that不从当任何成分是不是可以省略?宾语从句一般由什么词引导,

如果用that意思就是
我们还没讨论我们要摆放新家具
（意思不通顺吧）
用where的意思就是
我们还没讨论在哪里摆放这些新家具

关于宾语从句,请看以下总结：
“引导词”、“词序”、“时态”是学好“宾语从句”的三要素.
一、引导词：
“that”、“if(whether)”、“疑问词”是宾语从句的三类引导词,它们一定要放在宾语从句的最前面.
1、引导词that用来连接宾语从句是陈述句的句子,在口词或非正式文体中that常常省略.例如：
I hear (that) he will be back in a week．
He said (that) he felt terrible．Can’t you see (that) I’m a bird?
2、引导词if(whether)用来连接宾语从句是一般疑问句的句子.例如：
I want to know…Does he live here?
→I want to know if (whether) he lives here．
He asked …,“Has your father come back?”
→He asked me if (whether) my father had come back．
Could you tell me…? “Will Tom be back soon?”
→Could you tell me if (whether) Tom will be back soon?
3、如果宾语从句是特殊疑问句,那么特殊疑问句的疑问代词或副词就成为宾语从句的引导词.例如：Why is the bus late? Can you tell me?
→Can you tell me why the bus is late?
How can I get to the station? Could you tell me?
→Could you tell me how I can get to the station?
Why did he try to fool us? Can you tell me?
→Can you tell me why he tried to fool us?
二、词序
词序是指宾语从句中的词序,不可用疑问句的词序,而应用陈句的词序.即：主语在前,动词在后.例如：Can you tell me where he is?
Do you know why they didn’t obey the rules?
三、时态
时态是指在宾语从句中主句的谓语动词要与从句中谓语动词保持一致.即主语如果是一般现在时,那么从句可以用所需的任何时态；主句如果是一般过去时,那么从句也要用相应的过去时的某一种时态.例如：
She said they would go there by air．
Can you tell me when the meeting will start?
学了宾语从句的三要素,就能准确无误地掌握宾语从句.但还要提醒大家注意以下几点：
1、think、believe等动词引导的宾语从句,从句如含有否定意义,通常否定主句中的谓语动词.例如：
不说：I think he isn’t good at English．
而说：I don’t think he is good at English．
2、在主句动词和连接之间可插入其它成份.例如：
He told the girl that she was mistaken
3、连接代词或副词除连接作用、引导一个宾语从句外,还在从句中充当一个成份.因而不可省略.例如：
The teacher asked who could answer the question．
另外,在用what引导的宾语从句中,支配what的介词一般要放在句末.
4、if(whether)引导的宾语从句中,虽然它们在从句中不充当任何成份,只起连接作用,但由于本身有意义（是否）,因而不可省略.例如：
Mike asked Tom if it rained here yesterday．
He asked his classmates if (whether) they missed him．
5、从句如表示“普遍真理”或“客观存在的事实”,不管主句是什么时态,从句都用一般现在时.例如：
The teacher told us that the earth is one of the sun’s planets．