Plain form — the four shapes
You've already met the plain form — the dictionary entry of every verb (たべる, のむ, 行く, 来る, する) — and you've used it inside bigger patterns (〜ことが できます, 〜たら, 〜と 思います). Today we put it on its own. Plain form has four shapes — present, past, negative, past-negative — and together they're how Japanese works outside the polite 〜ます register. With friends and family, in your own diary, in casual songs and TV — that's all plain form. Knowing the shapes is the half of the lesson; knowing when to switch is the other half.
The four plain shapes
Every verb has these four: present (dictionary), past (〜た), negative (〜ない), past-negative (〜なかった). For たべる: たべる / たべた / たべない / たべなかった. For のむ: のむ / のんだ / のまない / のまなかった. For 行く: 行く / 行った / 行かない / 行かなかった. The two irregulars: する → する / した / しない / しなかった. 来る → 来る / 来た / 来ない / 来なかった. (You already know 〜た from lesson 18 and 〜たら; you've seen 〜ない in lesson 20 inside と 思います.) Three of the four are old friends — today you just stop hiding them inside other patterns.
How to build 〜ない from ます-form
If you know the polite 〜ます form, here's the shortcut. る-verbs (the ones whose dictionary form ends in a vowel + る like たべる, 見る, おきる): drop る, add ない → たべない, 見ない, おきない. う-verbs (the rest, ending in う / く / す / つ / ぬ / ぶ / む / る): change the final u-row syllable to its a-row partner, then add ない. のむ → のまない, 行く → 行かない, はなす → はなさない, あう → あわない (note: う → わ, not あ, for verbs ending in う). The two irregulars are listed above. Past-negative is just 〜ない → 〜なかった: たべなかった, のまなかった, 行かなかった.
Adjectives and nouns in plain form
い-adjectives are already plain when they don't have です on the end: さむい / さむかった / さむくない / さむくなかった. Just drop です. な-adjectives and nouns use だ / だった / じゃ ない / じゃ なかった (you met だ and だった in lesson 20). しずか だ / しずか だった / しずか じゃ ない / しずか じゃ なかった. がくせい だ / がくせい だった / がくせい じゃ ない / がくせい じゃ なかった. In casual conversation the final だ is often dropped — you'll hear しずか？ ('quiet?') with no だ at all — but writing it is always safe.
When to use plain — and when not to
Plain form is the default with friends, family, classmates your own age, kids, and pets. 〜ます is the default with strangers, in shops, with teachers and bosses, and any time you're being polite. Mixing within one conversation is fine — Japanese speakers slide between registers depending on who they're addressing. A safe rule for learners: stay polite (〜ます) until someone close to you uses plain form to you first. If your Japanese friend says きのう なに した？ ('what'd you do yesterday?'), it's an invitation to drop into plain. If you're not sure, 〜ます is never wrong.
Casual sentence shape
Plain-form sentences often drop the polite ending entirely: きのう、すし たべた。 = 'I ate sushi yesterday.' (No を, no です, just the past-form verb at the end.) Particles are often dropped in casual speech — わたし、行く instead of わたし は 行きます. Questions usually skip the か and just rise in pitch: 行く？ = 'are you going?' rather than 行きますか. The plain version of です is だ, and at the end of a casual sentence you often hear 〜だよ, 〜だね for emphasis: おいしい よ！, しずか だ ね。. For now, focus on building the four verb shapes — the casual sentence-final particles can come later.
Where you've already seen plain form
Plain form isn't only for casual speech — it's required inside a lot of polite patterns too. 〜ことが できます (lesson 15) takes plain dictionary. 〜時 and 〜前 に / 〜後 で (lesson 18) take plain. 〜たら (lesson 19) is built on 〜た. 〜と 思います (lesson 20) takes a full plain sentence inside. So learning plain pays double: it's the casual register and the engine room of polite Japanese. Practice the four shapes for every verb you already know.
