By definition, parts of speech fall into the category of "language arts," which is...
...essentially the study of how we use words to create and to decipher meaning. In general, schools divide language arts into four broad fields: reading, writing, speaking, and visual literacy.
All parts of speech are used in the writing process. Specifically, articles are perhaps seen as the least important part of a sentence, but there is no doubt that they provide valuable assistance in clearly understanding the written word. An "article" is defined as...
...any member of a small class of words…that are linked to nouns and…typically have a grammatical function identifying the noun as a noun rather than describing it.
[Articles] precede a noun or a noun phrase in a sentence.
In other words, an article is a small word that is placed in front of a noun, but does not describe it, and therefore, is not an adjective. In English, the primary articles are a, an and the. These articles can be definite or indefinite, and this important distinction lends to the understanding of a sentence, idea, etc.
The indefinite articles are a or an. ("An" is used before a word beginning with a vowel: a, e, i, o, u...and sometimes y.) The indefinite article can refer to any thing: a dog or an apple.
The definite article (the) refers to a specific thing: the dog or the apple. In this case, there is a restriction with regard to the noun—it is not just any dog or apple, but refers to a specific one. Though the article is small, it has a great deal of bearing upon the meaning of a sentence: if you write a mystery and a witness says he saw "a" man, it could be anyone.
The preposition is a much more sophisticated part of speech.
Prepositions work in combination with a noun or pronoun to create phrases that modify verbs, nouns/pronouns, or adjectives. Prepositional phrases convey a spatial, temporal, or directional meaning.
Often prepositions will covey movement or direction, as with the following example:
Over the meadow and through the woods...
Some prepositions are:
aboard, about, above, across, after, against, along, amid, among, around, at, before, behind, below, beneath, beside, between, beyond, by, down, during, except, for, from, in, into, like, near, of, off, on, onto, out, over, past, since, through, throughout, to, toward, under, underneath, until, unto, up, upon, with, within, without