Here are some possibilities for what you might be asking about:
1. Redundant Subject:
* This happens when a sentence includes two subjects that refer to the same thing. For example: "The dog, he ran down the street." The second subject "he" is redundant because it's already covered by "the dog". This is grammatically incorrect.
2. Compound Subject:
* This refers to two or more subjects joined by a coordinating conjunction (like "and", "or", "but"). For example: "The cat and the dog played together." This is grammatically correct.
3. Subject-Verb Agreement:
* This refers to the rule that the verb in a sentence must agree with the subject in number. For example: "The dog runs" is correct because "dog" is singular and "runs" is singular. "The dogs run" is also correct because "dogs" is plural and "run" is plural.
4. Misunderstanding of Subject-Object:
* Sometimes, people confuse the subject (the person or thing performing the action) with the object (the person or thing receiving the action). For example, in the sentence "The cat chased the mouse," "the cat" is the subject and "the mouse" is the object.
To clarify your question, please provide more context or an example sentence. I'd be happy to help you understand the grammatical concept you're asking about.