When is a plural verb not a plural verb?
When it’s the subjunctive form. Compare these sentences.
“If I was drunk at the time I made the offer, then it would be void.”
“If I were drunk at the time I made the offer, I never would have made it.”
“Were” in the second sentence is not the plural for “was” (we were), it indicates that the situation may or may not have been true. This is indicated in writing trough a verb form called the subjunctive mood. And the proper form of that verb with “I'” is “were.” It is the difference between “if I did” (definitely) and “if I did” (and I may not have).