What are Adjectives?
If you just use the word "car" in English, it’s not clear to your listener which car you mean, without some sort of context. However, if you say "the small, blue, broken car" instead, you give a much clearer picture as to which car you mean. Words such as small, blue, and broken are called adjectives. They are words that describe nouns, which are people, places, or things. These descriptive words exist in Japanese as well, and can be very useful, just as they can in English.
Types of Japanese Adjectives
There are two types of Japanese adjectives, -i adjectives and -na adjectives. They are written with a dash before them because they indicate the ending used on the adjective. The distinction is made when the adjective describes a noun, as shown by the following example.
shizukana hoteru (a quiet hotel)
furui hoteru (an old hotel)
The first adjective, shizuka, is followed by the na in order for it to describe hoteru. The second adjective, furui, is followed by i for the same purpose. When learning a new adjective in Japanese, it is important to see it used to describe a noun, because by itself, sometimes you cannot tell. Typically, -i adjectives are written with the final i in all contexts, conjugations aside. However, -na adjectives are often written without the final na when they are by themselves; thus the need to see a noun in order to determine the group of the adjective.
Things to Watch For
Some of the more common adjectives can be quite tricky, such as kirei (clean or pretty). At first glance, you may think this is an -i adjective, because it has a final i. However, look at it used in the following sentence:
Are wa kireina kutsu desu. (Those are pretty shoes)
Between the adjective kirei and the noun kutsu, we clearly see the na, indicating that this is, in fact, a -na adjective. There are many adjectives like this in Japanese, so it is important to see the usage of each when you first learn the word.
Another case comes with adjectives such as ookii (big) or chisai (small). These two adjectives, and some others like them, can be used as both -i and -na adjectives, as seen in the following example:
chisai hon (small book)
chisaina hon (small book)
There is no difference in meaning between the two, and choosing which form to use often boils down to personal preference, as well as dialect. For foreigners, it is typically best to pick one of the two and stick with it, until you are comfortable making the distinction.
Why does it Matter?
You may find yourself wondering, "Ok, so what if I use the wrong form? Won’t I be understandable either way?" This is a fairly reasonable question, especially from a native English speaker. After all, in English, our adjective groups are quite similar, and as a result, even native speakers constantly mix them up. While it may be proper English to say that one person is smarter than another, you would not be confused by someone who said that one person is "more smart" than another, even though it is incorrect.
In Japanese, however, adjectives play a larger role in the overall sentence structure. Depending on where you are in your studies, you may not have encountered it yet, but you will find that in Japanese, adjectives actually conjugate, just like verbs, to show affirmative, negative, and temporal differences. Due to this fact, -i adjectives and -na adjectives actually behave quite differently, and using the wrong form for one can be quite confusing, especially when you are in a different tense for your adjective.
In the end, it is a very small, yet important point to remember about the Japanese language. Simply using each new adjective you learn in a sentence can make a world of difference in your studies, and will greatly help you to distinguish between the two groups.