None of the sentences provided correctly use a compound subject. The correct way to phrase these would be:
- "My brother and I frequently visit the park."
- "Margo and they prefer swimming over running." (This sentence is awkward; "Margo and I prefer swimming over running" would be clearer.)
- "His father and they are preparing breakfast." (This should also be "His father and I are preparing breakfast.")
- "Elissa and she are studying for an exam."
A compound subject is correctly formed when using the subject pronouns in the nominative case (I, you, he, she, it, we, they). If you have a specific context or requirement, feel free to share!