Let's see how to use adjectives in simple Japanese sentences! All examples are in the present tense and use basic particles: は, が, か, に, で, and を. Try reading and listening to each sentence!
ねこはかわいいです。
neko wa kawaii desu
Cats are cute.
ねこは means "cats" indicating that the sentence is about the cat. かわいい is an い-adjective, meaning cute. The sentence would convert to "As for cats, they are cute." い-adjectives can be used directly before nouns or before です without modification.
いぬがおおきいです。
inu ga ookii desu
The dog is big.
いぬが introduces "the dog" as the subject of the sentence using the subject marker が. おおきい is an い-adjective, meaning big. No changes are needed to use い-adjectives with です.
このほんはおもしろいです。
kono hon wa omoshiroi desu
This book is interesting.
このほんは means "this book (close to me)" indicates that the subject of this sentence is the book. おもしろい is an い-adjective, meaning interesting. い-adjectives like this remain unchanged before です.
きょうしつでたかいいすをみます。
kyoushitsu de takai isu o mimasu
I see a tall chair in the classroom.
きょうしつ で means "in the classroom", indicating the location using the で particle. たかい is an い-adjective meaning tall, and it modifies the noun いす (chair) which comes right after it. The object marker を indicates that the chair is what is being seen. This sentence follows a more complicated structure: place + で + adjective + noun + を + verb.
きょうはビデオゲームをします。
kyou wa bideogeemu o shimasu
Today I will play a video game.
きょうは means "today" and marks the topic. ビデオゲームをしました uses the verb する in the future polite form to say "play a video game."
へやはしずかです。
heya wa shizuka desu
The room is quiet.
へやは sets "the room" as the topic. しずか is a な-adjective which means quiet. When used at the end of a sentence with です, you do not use な.
きみがげんきですか。
kimi ga genki desu ka
Are you well?
きみが marks "you" as the subject with が. げんき (Healthy/Energetic) is a な-adjective used as a predicate here with です, so な is not used. The sentence ends in か to form a question.
こうえんにきれいなはながあります。
kouen ni kirei na hana ga arimasu
There are pretty flowers in the park.
きれい (pretty) is a な-adjective. Because it comes directly before the noun はな (flowers), it uses な: きれいなはな. This follows the standard rule for attributive use of な-adjectives.
せんせいはゆうめいなひとです。
sensei wa yuumei na hito desu
The teacher is a famous person.
ゆうめい (famous) is a な-adjective. It comes directly before the noun ひと (person), so な is inserted: ゆうめいなひと (famous person). な-adjectives require な in this position.
きょうしつでべんりなものをつかいます。
kyoushitsu de benri na mono o tsukaimasu
I use a useful thing in the classroom.
べんり (convenient/useful) is a な-adjective. Because it directly modifies the noun もの (thing), な is required: べんりなもの (convenient/useful thing). The structure includes a location marker で and object marker を.