請問這句that名詞子句的用法

應該是名詞子句當同位語的用法吧！
I think the reason is that we concentrate on serving food of the very best quality for a reasonable price.
(我認為原因在於我們專心在以合理價格提供品質最好的菜色這點。)

什麼樣的原因呢？只有reason這個字又說不清楚，勢必要再補充說明reason的內容。所以要給它加上：
The restaurant is successful.(餐館經營成功。) 來讓reason這個字有意義。
直覺上我們似乎會聯想到"形容詞子句"；補充說明不就是再去"形容"它嗎？但這裡我們發現，要使用形容詞子句的話，要補充說明的先行詞必須也出現在子句當中，兩者才有關連性可以合併在一起(這也是為什麼我們把連接主要句子與形容詞子句的那個詞稱為"關係代名詞"的原因！)。但是這邊補充說明的句子裡面並沒有出現reason這個字啊！所以它沒辦法用形容詞子句合併。
既然不是用形容詞子句，那剩下的選擇就是名詞子句啦！因為名詞是可以以同位語型式當受詞的受詞補語的。這邊就是這種用法。
因此，把名詞子句用的that放進來後，接在reason後面，就完成了：
I think the reason that the restaurant is successful is that we concentrate on serving food of the very best quality for a reasonable price.
(我認為餐館成功的原因在於我們專心在以合理價格提供品質最好的菜色這點。)
如果我沒記錯的話，像reason這種字好像是叫做"抽象名詞"吧！類似的字還有：problem(問題)、fact(事實)、conclusion(結論)、idea(想法)、news(消息)、possibility(可能性)、theory(理論)、question(問題)...等等；一下子也想不到太多，只有想到這些。(小弟個人不喜歡記太多有的沒以的文法名詞；可能是我自己懶的關係，而且也覺得沒什麼必要弄得那麼累...) 你其實也可以去查市面上的文法書，我想應該會人把這些字都整理出來吧。

2009-04-23 18:11:33 補充：
名詞是可以以同位語型式當受詞的受詞補語的<< 在此更正這句話；非受詞而是主詞。the reason一直到最後面都是名詞子句，當think的受詞。但the reason本身在這個名詞子句裡面是當主詞。不過the reason後面的"the restaurant is successful"應該不是形容詞子句；還是解釋成同位語會比較好！

I was about to answer this question but saw that j28678 has already provided pretty good answer. Anyway, just for your reference, this is my answer. http://tw.myblog.yahoo.com/elisa-world/article?mid=648

2009-04-23 15:33:05 補充：
It is too long to post my answer in the comment section. Please refer to the link I provided.

I think {[the reason the restaurant is successful] is [that we concentrate
on serving food of the very best quality for a reasonable price]}.
我認為這間餐廳成功的原因是我們專注於以合理的價格提供顧客最優質的食物
1. {(that) the reason the restaurant is successful] is that we concentrate
on serving food of the very best quality for a reasonable price}. 這整句是名詞子句作為 think 的受詞, the reason 前面的關係代名詞 that 被省略掉. 而這名詞子句又包含了兩個子句.
2. the reason 是名詞子句的主詞, the restaurant is successful 是形容詞子句, 修飾 the reason. is 是名詞子句的動詞. that we concentrate on serving food of the very best quality for a reasonable price 是第二個名詞子句, 作為主詞 the reason 的補語.

I think [the reason the restaurant is successful] is that we concentrate
on serving food of the very best quality for a reasonable price.
這裡的the restaurant is successful是一個名詞子句做reason的同位語．reason是後面動詞is的主詞．全句也可以寫成　
I think that the reason why the restaurant is successful is that we concentrate on serving food of the very best quality for a resonable price.
這是一個complex sentence內有三個附屬子句．