YOU SAID:
She relieved herself, so she could relieve herself.
INTO JAPANESE
彼女は用を足したので、用を足すことができた。
BACK INTO ENGLISH
She had relieved herself so she could relieve herself.
INTO JAPANESE
彼女は用を足すために用を足した。
BACK INTO ENGLISH
She went to relieve herself.
INTO JAPANESE
彼女は用を足しに行った。
BACK INTO ENGLISH
She went to relieve herself.
That didn't even make that much sense in English.