The subject is a noun or pronoun that is doing something in the sentence. Nouns are persons, places or things. Pronouns are words like he, she and it. Subjects can be simple or complex, depending on whether there are descriptive modifiers present. The easiest way to determine the subject is to find the verb, or "action" of the sentence, and ask what is doing that action. The actor is the subject. A subject can be as simple as "The boy."
The predicate contains the verb of the sentence. The verb is the action that is being undertaken by the subject. The action can be taking place in the present, in the past or in the future. These are called verb tenses. Verbs can also have modifiers or exist in clauses, but a simple predicate is just the verb. "The boy walked" is a complete sentence, with "walked" being the predicate.
An object sometimes follows the verb. It receives the action of the verb in some way. There are two types of objects, direct and indirect. A direct object is the "what" of the action. An indirect object essentially "receives" the direct object; answering to whom or for what the direct object is intended. The sentence "the boy walked the girl home" contains both a direct and indirect object. "The girl" is the direct object because the boy is walking her somewhere. "Home" is the indirect object because it tells where they are walking.
A modifier is a word or clause that adds information about another word. Adjectives modify nouns and adverbs modify verbs. While adverbs can be found almost anywhere in a sentence, adjectives almost always appear directly in front of the nouns they describe. "The tall, young boy quickly walked the pretty girl home" contains both adjectives and an adverb. "Tall" and "young" are adjectives that describe the boy, and "pretty" describes the girl. "Quickly" is an adverb that gives more information about the verb "walked."