Understand the difference between subjective and objective pronouns. Subjective pronouns are the subject or topic of the sentence. Subjective pronouns are I, we, you, he, she, it and they. Objective pronouns are the target of the verb, meaning the person or thing to which the action of the verb is happening. Me, us, you, him, her, it and them are objective pronouns.
Learn the definition of a compound subject. A compound subject happens when there are two or more different nouns or subjects in a sentence. In the sentence, "Jane watches the dog eat," Jane is the singular subject of the sentence. In the sentence, "Jane and John watch the dog eat," Jane and John make up the compound subject of the sentence.
Replace noun subjects with subjective pronouns only. Objective pronouns cannot replace nouns in the subject of a sentence. In the sentence, "Jane and John watch the dog eat," an example of replacing the compound subjects with subjective pronouns would be using "she" for Jane and "he" for John. The new sentence would correctly read, "She and he watch the dog eat."