To construct a sentence, you need a noun and a verb, so if you write "The bear roared," you have written a sentence. The word "bear" is a noun and the subject of your sentence, and "roared" is your action verb. If you write "She roared," it, too, is a sentence. The word "she" is a pronoun; a pronoun is a word that substitutes for a noun. Other pronouns are "he," "she," "they," "we," "his" and "her."
If you add the words "whoa" and "angry" to your sentence -- "Whoa! The angry bear roared!" -- then you have introduced an interjection (whoa) and an adjective (angry) to your sentence. Interjections express emotions, such as surprise, and are usually followed by an exclamation mark. The most common job for adjectives is describing nouns. They usually precede the noun or pronoun, but not always. Other descriptive words or adjectives for the bear could be gentle, black or hungry.
If you add an adverb to your sentence, it could read "Whoa! The angry bear roared noisily!" Adverbs usually end with "-ly," and they are used to describe or modify the verb or action word. To confirm if you have added an adverb to your sentence, ask yourself if "noisily" describes how, when, where or why the bear roared. Some words that end in "-ly," such as "lovely," are not adverbs but adjectives, and adjectives are used to modify or change nouns or pronouns.
To continue building your sentence, you might want to add a preposition. This part of speech joins nouns, pronouns and phrases to other parts of your sentence. Words such as "behind," "under," "during" and "through" are prepositions. If your sentence is "Whoa! The angry bear roared noisily behind me!" you have now included the preposition "behind" in your sentence.
The conjunction is the last word in your list of parts of speech. It is used to join two words, phrases or sentences together. Some examples are "but," "because" and "or." If you write "Whoa! The angry bear roared noisily behind me, and I almost jumped out of my shoes!" you have added the conjunction "but." You have now constructed a sentence using all the parts of speech and you are ready to effectively communicate.