October 15 th is my birthday,which I will never forget.
这里which代表前面整句话,也就是October 15 th is my birthday
如果逗号去掉,那么which就代表birthday,作宾语
I will never forget the birthday
when的话,=on which
October 15 th is my birthday (when/on which) my father gave me a present
my father gave me a present on my birthday
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