YOU SAID:

the hippo i sat on was my brother's hippo maybe he'll be mad at me

INTO JAPANESE

私が座ったカバは私の弟のカバだったかもしれない

BACK INTO ENGLISH

The hippo I sat on might be my brother's hippo

INTO JAPANESE

私が座ったカバは私の弟のカバかもしれません

BACK INTO ENGLISH

The hippo I sat on might be my brother's hippo