普通形
The plain form of verbs, adjectives, and nouns — the foundation of every N4 patternmachine translation
FormationVerbs, adjectives, and nouns in four forms: present, negative, past, past negative
The plain form (普通形) is the style that doesn't use です・ます. For verbs, you already have all the ingredients from N5: the dictionary form (行く), the ない-form (行かない), the た-form (行った), and 行かなかった for the past negative. These four together make up the plain-form chart for verbs.
い-adjectives just drop です: 高い・高くない・高かった・高くなかった. Nouns and な-adjectives replace です with だ: 元気だ・元気じゃない・元気だった・元気じゃなかった — the details of だ are in the だ／じゃない lesson.
Why does the plain form matter so much? First, it's the casual style used with friends and family. Second — and more important for the N4 exam — nearly every new grammar pattern connects to the plain form: 〜と思う, 〜ので, 〜かどうか, noun-modifying clauses… Get this conjugation chart down solid before moving on; every lesson that follows stands on it.
Examples
明日は学校に行かない。
I'm not going to school tomorrow.
昨日の映画はとても面白かった。
Yesterday's movie was really interesting.
彼は日本語の先生だ。
He is a Japanese teacher.