Can you tell me ____ the toy I bought for you is under the table. 為什麼不能用that, 還有間接問句的文法詳細，謝謝。?

Can/could you tell me [connective word] ....?
Would you mind telling me ....?
Do you know ...?

The above sentences are indirect questions 間接問句 which are a way of being polite. They are very common in English, especially when you're talking to someone you don't know.

[connective word] includes:
- interrogative pronouns (who, whom, whose, which, what)
- interrogative adverbs (why, where, when, how)
- conjunction (if or whether) for yes/no questions

The real question actually follows the polite expression "can you tell", called direct question.

Possible direct questions for the example:
1. Why is the toy I bought for you under the table?
2. Is the toy I bought for you under the table?

Respectively, the indirect questions would be:
1. Can you tell me why the toy I bought for you is under the table?
2. Can you tell me if the toy I bought for you is under the table?

Note that there is no inversion for the real question part. "that" cannot be used as the connective word in the above example as it is not a question word to form the direct question.

**題外話** Imperative sentence requesting for confirmation or confession :
Tell me that xxxxxxxxxxxxx.
Sometimes, you can say it in a more polite way, such as:
Can you tell me that xxxxxxxx?

eg. Tell me you love me. Tell me you are wrong.

應該是用"why".

Can you tell me why the toy I bought for you is under the table.

= tell me why the toy I bought for you is under the table.

是屬於"明知故問"假裝為問句的命令句.

That在文法上當然也可以.可是意思就不太一樣:

Can you tell me that the toy I bought for you is under the table?
= Please tell me that the toy I bought for you is under the table!
= 拜託告訴我 那隻玩具 就在桌下.
= 要是那隻玩具 不就在桌下 的話 就會有死人的!

適用的情景有:

很重要很有意義的玩具 丟掉了 到處找不到 就只剩下桌子下面還沒找.
媽媽就可以對北鼻 說: Can you tell me that the toy .... under the table?

這句本身有點"奇怪"...
Can you ...應該是一句"問句？，但是內容或句後面卻是好像肯定已經知道或確知物件的位置是在枱底(下面)"is under the table.
因此只能推測這是一句"疑問句"，問者是查問一個假設或認為要確定的答案(是否正如假設)，因此只有一個連接詞
Can you tell me whether the toy I bought for you is under the table.
才能配合前文和後理。

why------=relative adverb----=the reason----=Tell me why------>Do you know why(the toy I bought for you) is under the table ?