YOU SAID:
I've never done this, what are you on about
INTO JAPANESE
こんなことしたことないよ、何してるの?
BACK INTO ENGLISH
I've never done this before, what are you doing?
INTO JAPANESE
こんなことしたことないんだけど、何してるの?
BACK INTO ENGLISH
I've never done this before, what are you doing?
That didn't even make that much sense in English.