<Was/were> you reciting at the concert?As the subject is singular, I guess the verb will be singular; in this case "was". Am I right? Please advise?

You , the subject, can be singular or plural. In other words, it can refer to one person or many people. However, that has nothing to do with the fact that was is never used with it.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.