When two subjects are contrasted in a sentence, the verb agrees with the affirmative subject:
He, not I, am responsible for this terrible mess. ✗
He, not I, is responsible for this terrible mess. √
She, not you, always complain about the stench coming from the drains. ✗
She, not you, always complains about the stench coming from the drains. √
They, not he, was first to report on the scandal. ✗
They, not he, were first to report on the scandal. √
I, not he, has suffered the most since mother’s death. ✗
I, not he, have suffered the most since mother’s death. √
