MUST 的用法

must 後面係咪一定要跟 infinitive?
XXX must + verb <--------
Ans: 不一定

還有以下寫法﹕
1. XXX must + have + seen/past participle <--------
have + participle <--------- It implies "THE PAST".
This action is complete.

Example: Your father told you that he was going to meet Simon.
Now you are talking to your father. >>> Daddy, you must have met Simon. How was he?
must + have + met === a complete action
must ------------- very very sure

About your sentence:
I must have seen this book (before). <<< 有人質疑你是否見過這本書，而你又非常肯定有的時候，你跟那人爭辯時，就會用這句話來回應他。
(Note: see ; read <-------- 意思有分別的)

2. XXX must + be + adjective/adverb/verb+ing/article
She 一定是 must be happy. (happy係adjective)
She 一定是 must be really happy. (really係adverb)
She 一定是 must be a good teacher. (a係article)
She 一定是 must be talking to Mr Lee now. (talking係verb+ing) <<< ﹙這個用法有推斷的含意。﹚說這話時，你雖並沒有親眼目睹事情的發生，但卻十分肯定事情正在這樣發生。

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must 和其他 modal verb 後面第一個(只係第一個)動詞或者助動詞係要用原字
即係不可以用 is / am/ are/ was/ were, 要用 be
不可以加 s, es, ed, ing 等等

e.g.
You must be crazy. (are錯)
She must have seen this book before. (has seen 錯)
He must keep quiet. (keeps 錯)
She must have been lying. (has 錯)

1)Must is used for expressing an obligation or a duty.
e.g.He must go home immediately.

*Words that sometimes function like modal include "must"*