PART I
A good way to begin understanding basic sentence structure in English is to identify the eight traditional parts of speech, as defined by dictionary.com.
Noun - any member of a class of words that can function as the main or only elements of subjects of verbs (A dog just barked), or of objects of verbs or prepositions (to send money from home), and that in English can take plural forms and possessive endings (Three of his buddies want to borrow John's laptop). Nouns are often described as referring to persons, places, things, states, or qualities.
Pronoun - any member of a small class of words found in many languages that are used as replacements or substitutes for nouns and noun phrases, and that have very general reference, as I, you, he, this, who, what. Pronouns are sometimes formally distinguished from nouns, as in English by the existence of special objective forms, as him for he or me for I, and by nonoccurrence with an article or adjective.
Verb - any member of a class of words that function as the main elements of predicates, that typically express action, state, or a relation between two things, and that may be inflected for tense, aspect, voice, mood, and to show agreement with their subject or object.
Adverb - any member of a class of words that function as modifiers of verbs or clauses, and in some languages, as Latin and English, as modifiers of adjectives, other adverbs, or adverbial phrases, as very in very nice, much in much more impressive, and tomorrow in She'll write to you tomorrow. They relate to what they modify by indicating place (I promise to be there), time (Do your homework now!), manner (She sings beautifully), circumstance (He accidentally dropped the glass when the bell rang), degree (I'm very happy to see you), or cause (I draw, although badly)..
Adjective - any member of a class of words that modify nouns and pronouns, primarily by describing a particular quality of the word they are modifying, as wise in a wise grandmother, or perfect in a perfect score, or handsome in He is extremely handsome. Other terms, as numbers ( one cup; twelve months), certain demonstrative pronouns (this magazine; those questions), and terms that impose limits (each person; no mercy) can also function adjectivally, as can some nouns that are found chiefly in fixed phrases where they immediately precede the noun they modify, as bottle in bottle cap and bus in bus station.
Prepositions - any member of a class of words found in many languages that are used before nouns, pronouns, or other substantives to form phrases functioning as modifiers of verbs, nouns, or adjectives, and that typically express a spatial, temporal, or other relationship, as in, on, by, to, since.
Conjunctions - any member of a small class of words distinguished in many languages by their function as connectors between words, phrases, clauses, or sentences, as and, because, but, however.
Interjections - any member of a class of words expressing emotion, distinguished in most languages by their use in grammatical isolation, as Hey! Oh! Ouch! Ugh!
PART II
In its most basic form, a sentence is comprised of a subject (i.e., noun or pronoun) and verb (with the verb usually following the subject), as in:
"She jumped."
In this example, She is the subject because it shows who's performing the action. Who jumped? [She] jumped. A subject typically indicates what the sentence is about or who/what performs the action.
Now, jumped is the verb in the sentence because it shows action. What is she doing? She is [jumping].
Naturally, I've given you a very basic sentence, but one that is complete because it contains both subject and verb. Keep in mind, though, that you can construct very complex sentences with the addition of other components, such as dangling modifiers, prepositional phrases, clauses, etc. Still, even complex sentences must always have a subject (or implied subject) and verb.
For more help, refer to: http://grammar.about.com/od/basicsentencegrammar/a/basicstructures.htm