If two or more subjects are connected by a conjunction, the subject is compound. Compound subjects may be singular or plural. They can be used in simple sentences, compound sentences, complex sentences or compound-complex sentences, depending on whether the compound subject is part of a sentence containing a single independent clause, two independent clauses or a mixture of independent and dependent clauses.
Compound Subjects and Simple Sentences
A compound subject is usually plural if the conjunction used adds several singular subjects together. "Sam, Bert and I" is plural because the "and" indicates that the subjects are added together. "Sam, Bert or I" is a singular compound subject, because the conjunction "or" separates the subjects into three individual parts. In a simple sentence, the compound subject is part of one independent clause. For example, "Sam, Bert or I will grill the hamburgers" is a simple sentence with a compound subject.
An independent clause is one that contains a subject and verb and functions on its own as a sentence. A dependent clause also contains a subject and verb, but it is not a complete sentence because it does not contain a complete thought. A compound sentence contains two independent clauses joined by a conjunction. A complex sentence has one dependent and one independent clause. A compound-complex sentence contains two independent clauses and at least one dependent clause. Any of these sentence types may contain a compound subject.
"And," "or" and "nor" are the coordinating conjunctions used to form compound subjects. "And" usually forms plural subjects, while the others form singular subjects. When a compound subject contains both singular and plural parts, whether the verb is singular or plural depends on which subject is nearest the verb. "Max and Leo or I am driving" or "I or Max and Leo are driving" are examples of the differing verb agreement.