In Japanese, there are two kinds of adjectives: い (i) and な (na) adjectives. What's the difference? Like る and う verbs, い-adjectives end in い. な-adjectives are different though, they end in な, but only when they modify a noun.
Take for example the adjective げんき (genki = lively, energetic). By itself, it is an adjective. However, if you modify a noun, な is added at the end of げんき.
げんきながくせい (genkina gakusei) = energetic student
Some example of な adjectives are: すき (suki = likeness), だいすき (daisuki = loveable), しずか (shizuka = quiet), ハンソム (hansomu = handsome), ひま (hima = not busy; free)
い-adjectives are simple in the way that they always end in い even when they modify a noun. Let's use the adjective おもしろい (omoshiroi = interesting).
おもしろいがくせい (omoshiroi gakusei) = interesting student
As you can see, い is still used at the end of the adjective and there is no further need to change it. Some examples of い-adjectives are: ちいさい (chiisai = small), あつい (atsui = hot), たのしい (tanoshii = fun), かわいい (kawaii = cute), むずかしい (muzukashii = difficult)
Like always though, there are strange exceptions. Some adjectives can be used for both, like おおきい (ookii = big). おおきい is an い-adjective, as it ends in い. However, it can also use なwhen it modifies nouns, in which case it would be おきな (okina). The meaning is still the same and both forms of usage are considered correct.
おおきいがくせい (ookii gakusei) = おきながくせい (okiina gakusei) = big student
Now that you know about adjectives, hopefully you'll be able to identify them when you hear them! You've probably heard many Japanese adjectives before, but if you haven't, you can now recognize most words if they are い- or な-adjectives.
Be careful though! Some adjectives that end in い are in fact な adjectives! For example, きらい (kirai = dislike) ends in い, but is a な adjective, and is きらいな when it modifies nouns. This was a small, but fatal mistake that almost got me into a lot of trouble in Japanese class. Take it from me and make sure you know which adjectives are い- or な-adjectives!