YOU SAID:
she could come to the party because she was very busy
INTO JAPANESE
彼女はとても忙しかったのでパーティーに来ることができました
BACK INTO ENGLISH
She was so busy that she could come to the party
INTO JAPANESE
彼女はとても忙しかったのでパーティーに来ることができた
BACK INTO ENGLISH
She was so busy that she could come to the party
You've done this before, haven't you.