it was you not he that __to blame for the fire.
谓语用单数还是复数
1.先还原成：you not he/him are/were to blame for the fire.
2. not “而不是”的意思.表示排出.所以本句的主语是you.
3. It was you not he/him that were to blame for the fire.
强调句式.强调主语you.