English is a complicated language.  It's rather annoyingly difficult in many respects, and somewhat simplified in others.  Many people have difficulty with English spelling.  The grammar has it's ups and downs, but here is some help.  This series is for those who would like to brush up on or learn about grammar and grammar terminology.

English articles, adjectives, and adverbs only have one form, with the exception of some demonstratives. In the old days, they used to have cases and would change depending on the nouns they describe (except adverbs since those don't modify nouns).  These days, they don't really change.  Both articles and adjectives come before the noun they describe.  Adverbs usually have some free range as to where in a sentence they can occur.

There are two major articles, definite and indefinite.  The definite is the word "the" and it occurs in cases where the noun is already known to or in some way.  It is often talking about a specific things (take the King, or the President).  The indefinite article is the word "a" and it doesn't really have a plural, but sometimes the word "some" or "a few" is used in that way.  This is used in cases where the noun is not previously known, and it is not a specific thing (take a man, a dog). Where these two are used depending on the case, and sometimes it is omitted or changed depending on what the noun is, the sentence, or idioms with the word.  In English it is common for abstract nouns not to get articles at all (truth, liberty, etc).  In Greek, I believe that even names get an article, unlike English and a good few other languages.  There are different rules for where and when they occur, depending on your grammar book.

Adjectives come before nouns in a sentence (as opposed to most romance languages).  Adjectives describe nouns.  This is common in many Germanic languages.  If there is an article the adjective and all it's components go between the article and the noun (The fair maiden). In poetry one can put "so" in and place the adjective after the noun (the maiden so fair).  Many adjectives can stand alone in a "to be" statement or with helping verbs, called a predicate adjective by grammarians (The maiden is/looks fair).  Idiomatic expressions also employ adjectives in funny ways.  There are many endings to adjectives, some of which help one to pick them out in a sentence.  The endings <-y>, <-al>, <-ic>, <-ish>, <-ful>, are just a few.  There are also hidden ones and some new ones dubbed from nouns all the time.  <Lonely> is an adjective though it looks like an adverb, <good> could be an a noun (forces of good) or an adjective (the good man) although the meaning is changed slightly.
Demonstratives are something like both articles and adjectives.  Demonstratives can change due to number.  This and these are the definite forms, and that and those are the indefinite forms.
Possessive Pronouns are themselves half pronoun, half adjective (or article).  This is the only one which has a different form for the predicate and prepositional.  So you have a word like "my" (as in my pencil) and then you have "mine" (as in, the pencil is mine).  This goes on for all the possessive pronouns.  (It could also be argued that possessive nouns with the -'s ending are adjectives in themselves.  I would argue them as Genitive nouns as I did in the English Grammar: Nouns article.)

Adverbs are a little different than their noun-tied counterparts.  Adverbs are words which verbs, adjectives, other adverbs, as well as do a few other things.  Adverbs also have a tendency (especially those describing verbs) to be able to move around in an English sentence.  A sentence like, Tomorrow I go, can also be said, I go tomorrow, and be accepted as normal.  The ones that modify adjectives and other adverbs often go next to that which they describe.
The common way to change an adjective into an adverb is to add the ending <-ly>.  So a word like <quick> in, The boy is quick, turns into, The boy runs quickly.