What is an Adjective?
Adjectives are words that describe something or someone. Scruffy, purple, concerned, and special are all adjectives. They usually (but not always) come right before what they are describing. Here are some examples:
"A scruffy dog sat in the window." (Scruffy is the adjective, and dog is the thing being described.)
"She wore a purple shirt." (Purple is the adjective, and shirt is the thing being described.)
“The birthday seemed special.” (Special is the adjective, and birthday is the thing being described.)
Some adjectives describe qualities–spiciness, for example–that can vary in amount or degree. They often do this by changing form (usually by adding -er or -est): you might prefer your food spicier than others in your family do, or take pride in your ability to handle the spiciest hot sauce on the market. But adjectives can also be used with adverbs like more, most, less, very, slightly, etc. for this purpose. Beautiful is one example of an adjective whose degree can only be modified in this way; you could say that one painting is more beautiful than another, but you would not way that it is beautifuller.
Comparative and Superlative Adjectives
The question of whether you should modify an adjective with -er or -est or if you should use more or most is a tricky one. A very general rule (which has many exceptions) is that -er or -est may be used for adjectives of a single syllable (like round or flat), more or most are used for those with three syllables or more (like beautiful), and there is no clear rule for adjectives with two syllables. (Read more about modifying adjectives)
Other Roles of Adjectives
In addition to describing qualities of nouns and pronouns, adjectives can modify them by restricting or limiting their meanings. Take numbers, for example: “fifteen cupcakes,” “zero qualms.” Similarly, although they are often used as pronouns, this, that, these, and those can also be used as adjectives: “This article was very informative.”