Head noun (or pronoun) - the central word identifying the subject. 
Determiner - a word that introduces and limits the noun.
Adjective modifiers - one or more adjectives or adjective phrases describing the head noun.
Prepositional phrase postmodifier - a short phrase that follows the noun, giving extra detail. 
Relative clause - a short clause that provides more information about the noun.
Appositive - a noun phrase next to the subject that renames it.
Compound subject - two or more subjects joined together. 
Expletive (or dummy subject) - a placeholder word used when the real subject comes later. 
Pronoun case and clarity - a rule where the pronouns must be in the right form and clearly refer to someone. 
Agreement (number/person) - a rule where the subject's form (singular/plural) decides the verb form.