[請益] must be v.s must have been

[aphroditeyeh (新的開始)]

想請教大大一個問題，下列這兩句話哪一句才是對的呢?
His parents must be very happy since he won the first grade.
His parents must have been very happy since he won the first grade.
非常謝謝您!


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推 dashihito:must have been；must have Vpp→對過去的肯定推測。
→ dashihito:must be；must V原→對現在的肯定推測。
→ dashihito:看你覺得他爸媽是他得獎後是現在爽還是過去爽囉～
→ aphroditeyeh:謝謝!所以這裡的since如果解釋成自從，也可以用must
→ aphroditeyeh:must be嗎? 會不會涉及到要使用完成式表達一直高興?
→ priv:這邊的 since 是「因為」不會是自從
→ priv:基本上 must be 比較自然
→ priv:因為這是一個「推測」，而不是陳述
→ aphroditeyeh:Thank you so much :)