* Nouns: These are the most common type of subject (e.g., "The dog barked").
* Pronouns: These words stand in for nouns (e.g., "He barked").
* Noun phrases: These are groups of words that act as a noun (e.g., "The big, brown dog barked").
* Gerunds: These are verb forms ending in "-ing" that act as nouns (e.g., "Running is good exercise").
* Infinitives: These are verb forms preceded by "to" that act as nouns (e.g., "To sleep is the best thing right now").
* Clauses: Sometimes a whole clause can act as the subject (e.g., "What she said surprised everyone").
Important Note: While any of these can be the subject, they must function as a noun within the sentence. The subject is the person, place, thing, or idea that is doing or being something.