I am happy for you (on your behalf): difference between 為你開心 and 當妳開心？ Assume a friend earns a promotion, and you want to say, "I'm happy for you."

In English, there is a difference in meaning when you say:


I am happy because of you: suggests you do things that make me happy.
I am happy for you (or on your behalf): suggests something good happened in your life, and i share your joy.


In Cantonese, it seems like the way to say I am happy for you is 當妳開心。
為你開心 would mean I'm happy because of you. The Cantonese reply to a friend receiving a promotion would be 當妳開心.


Are these Cantonese meanings and interpretations correct?
If so, is it the same for Cantonese and Mandarin, or in Mandarin does 當妳開心 carry the same meaning as 為你開心?
In Cantonese, 戥你開心 is the most common way to say &quot;happy for you&quot; (not 當)

戥 http://www.cantonese.sheik.co.uk/dictionary/characters/8501/
(1). for (a person) | [華] 替, 為

As this entry stated, the counterpart in Mandarin is 替 or 為, as in 替你開心, 為你開心. All mean &quot;happy for you&quot;
Side note:
戥's meaning as a verb is 'to counter balance'.
Two unbalanced load of goods at two side of a 擔挑 is hard to carry . So you have to &quot;戥均&quot; (to make balance by counter balance  one side )
That's how '戥' acquired the meaning of &quot;for (a person)&quot;

A counter balance is something 'for' something else.

戥 someone happy is happy 'for' someone else


替 (in your stead)--&gt;替你開心 (Happy in your stead= happy for you)
為 (because of) --&gt; 為你開心 (happy because of you are in certain situation, e.g. 'graduating')