連綴動詞 後面可加adverbs?

I am really sure that this is a real diamond, not a fake.

在你這句子, really 是副詞 (adverb), 修飾個 sure (形容詞)
really = 確實地, 的確 〔用於加強語氣〕

I am sure = 我確定
I am really sure = 我真的確定
really 加強語氣

下列三個都是副詞, 同樣地可以修飾 sure

I am very sure..~ very (很﹐非常)
I am absolutely sure.. ~ absolutely (完全地; 絕對地)
I am completely sure..~ completely (完全地﹐徹底地)

She is very beautiful. (可以 用其他的副詞 absolutely. extremely 修飾 beautiful )

副詞放置連綴動詞 和 形容詞中間
linking verb + adverb + adjective

副詞還可以修飾動詞 (verb), 其他的副詞. 整體句子

He ran quickly.
~ quickly 修飾動詞 ran

He ran very quickly.
~ very 修飾副詞 quickly

Fortunately, all the passengers survived the plane crash.
~ Fortunately 修飾整句

副詞位置
許多的副詞可放置句子後面, 前面, 中間, depending on context and style
He turned round slowly.
Slowly he turned round
He slowly turned round.

但是你的句子型式 (sentence pattern)

subject + linking verb + adverb + adjective + ,,,,,,,,,,,,

重說:
副詞放置連綴動詞 和 形容詞中間
linking verb + adverb + adjective

千萬不可以書寫或者講
"I am sure really-........" ~ 巔倒是非,
這是初學英語者經常犯的錯誤, 錯了都懵然不知
really 放置 "I am sure" 後面似修飾句子 "I am sure"
應該是 I am really sure .. ......

"really" doesn't give more information about the adjective "sure'.
It is commonly used to emphasize an adjective.

I am sure really---"linking verb---adj---adv."
"really" is an adv giving us more information about the adj sure.
"really" adv., is used to say what is actually the fact or the truth about something (the diamond). What do you really think about a diamond? Tell me what really happened to the real diamond.They are not really fake but real.I can't believe it is really a real diamond.etc.
We may say that adverbs are words which modify verbs, adjectives and other adverbs. Hence really---->sure.(=adv. to adj.)
"that this is a real diamond' is a coordinate noun clause.
"not a fake" is a phrase.