Talk casually with friends: The plain register (普通体)

Mental Model: The Intimacy Mirror (Uchi vs Soto)

Japanese society operates on concentric social circles:

• Outer Circle (外 / Soto): Teachers, bosses, clients, strangers, and distant acquaintances. Requires polite forms (丁寧語(ていねいご) (polite language): です and ます) to maintain respectful boundaries.
• Inner Circle (内 / Uchi): Family, close friends, peers of the same age, and internal mental thoughts. Requires plain style (普通体(ふつうたい) (plain form)).
• The Deep Cultural Insight: Speaking in polite forms (です / ます) with your spouse, best friend, or sibling is not considered polite—it creates an artificial, chilling barrier that signals emotional distance!

The Verb Conversion Matrix

Every polite verb ending has an exact plain form counterpart:

• Present Affirmative: 〜ます → Dictionary form (辞書形(じしょけい) (dictionary form)): 行きます → 行く (“go”) · 食べます → 食べる (“eat”).
• Present Negative: 〜ません → 〜ない form: 行きません → 行かない (“not go”) · 食べません → 食べない (“not eat”).
• Past Affirmative: 〜ました → 〜た form: 行きました → 行った (“went”) · 食べました → 食べた (“ate”).
• Past Negative: 〜ませんでした → 〜なかった form: 行きませんでした → 行かなかった (“did not go”) · 食べませんでした → 食べなかった (“did not eat”).

Nouns and な-Adjectives: です Becomes だ

For nouns and descriptive stems, です transitions into the plain copula だ:

• Present: Affirmative: 学生です → 学生だ (“is a student”) · Negative: 学生じゃありません → 学生じゃない (“is not a student”).
• Past: Affirmative: 学生でした → 学生だった (“was a student”) · Negative: 学生じゃありませんでした → 学生じゃなかった (“was not a student”).

The Question Trap: Drop か and Raise Your Intonation!

The most dangerous pitfall for learners transitioning to casual speech is asking questions:

• In Polite Speech: Attach か to the verb: 食べますか。 (“Will you eat?”).
• In Casual Speech (The Interrogation Trap): Never say *食べるか to a friend! Attaching か to a plain verb sounds harsh, masculine, and aggressive—like a police interrogation or an anime villain.
• The Natural Casual Question: Simply drop か entirely and speak with a rising intonation: 今、暇？(いま、ひま？) (“Are you free right now?”) · これ、食べる？(これ、たべる？) (“Want to eat this?”).
• The Explanatory Question Particle (の): You can also soften the question by adding の: どこに 行くの？(どこに いくの？) (“Where are you going?”). For nouns and な-adjectives, insert な: 学生なの？ (“Are you a student?”).