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