I am happy for you (on your behalf): difference between 為你開心 and 當妳開心？
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')