Final answer:
Compound subjects are two or more subjects joined by a conjunction that share the same verb, usually necessitating a plural verb, except when they form a singular idea.
Step-by-step explanation:
Two or more subjects joined by a conjunction but having the same verb are called compound subjects. This occurs when the conjunction 'and' is used to join the subjects in a sentence, which usually requires a plural verb to agree with the compound subject. For instance, in the sentence 'The dog and the cat are friends,' 'the dog' and 'the cat' form a compound subject that agrees with the plural verb 'are.' However, there is an exception to this rule when the two subjects joined by 'and' represent a single idea or entity, and in such cases, a singular verb is used, as in the sentence 'Macaroni and cheese is my favorite meal.'
It is important to note the difference between complex sentences and compound predicates. A complex sentence contains one main clause and one or more subordinate clauses, while compound predicates refer to a single subject followed by two or more verbs or verb phrases connected by a conjunction.