What is the subject of the following sentence?: John and I are shopping at the mall.
The subject is "John and I," which is a compound subject.
The subject in this sentence is a "compound subject."
Compound subjects are formed from two or more subjects joined by a coordinating conjunction (and, both, or, neither, nor).
When the conjunction is "and," the verb is plural
(compound subject) (plural verb)
You can find out more about compound subjects here: