In short

Every verb, adjective and noun predicate has a plain form (普通形) with the same four cells you already know from ます: non-past, non-past negative, past, past negative. The plain form is how words are listed in a dictionary, how friends talk to each other - and, most importantly, the form a subordinate clause is built on, even when the sentence ends politely. Learn these four cells now; the next fifteen lessons all attach to them.

The rule

The polite ます-forms from lesson 7 belong at the very end of a sentence. Everything that happens inside a sentence - a time clause, a noun being modified, a thought being quoted - is built on the plain form instead. So this lesson is not “the casual register”: it is the structural gear of the whole language.

Step 1 - The dictionary form (辞書形) is the non-past affirmative and needs no work: 書く kaku, 食べる taberu, する suru, 来る kuru. This is the citation form - what you type into a dictionary, and what a textbook glosses.

Step 2 - The ない-form (ない形) is the non-past negative.

• Group II: drop る, add ない. 食べる -> 食べない tabenai, 見る -> 見ない minai.
• Group I: move the final -u kana to its -a row partner, then add ない. 書く kaku -> 書か + ない -> 書かない kakanai; 飲む nomu -> 飲まない nomanai.
• The う trap: a verb ending in う goes to わ, not あ. 買う kau -> 買わない kawanai, 会う au -> 会わない awanai. There is no ×買あない - the w is a leftover of older Japanese, and it is regular across every 〜う verb.
• Irregulars: する -> しない shinai, 来る -> 来ない konai (the kanji stays, the reading moves ku -> ko).
• ある is suppletive: its negative is simply ない nai. There is no ×あらない. But its animate partner いる is regular: いない inai.

Step 3 - The た-form (た形) is the past affirmative, and you already know it. It uses exactly the same sound changes as the て-form from lesson 13, with て -> た and で -> だ. Do not memorise a second table: 書いて -> 書いた, 飲んで -> 飲んだ, 行って -> 行った, 食べて -> 食べた, して -> した, 来て -> 来た.

Step 4 - The なかった-form (なかった形) is the past negative. Take the ない-form and treat that ない as if it were the い-adjective 高い: drop い, add かった. 書かない -> 書かなかった kakanakatta, 来ない -> 来なかった konakatta, ない -> なかった nakatta.

い-adjectives already work this way. 高い takai -> 高くない takakunai -> 高かった takakatta -> 高くなかった takakunakatta. The polite versions you learned in lesson 8 were just these plus です; strip the です and you have the plain form. いい is irregular as always: よくない, よかった, よくなかった.

Nouns and な-adjectives use the copula だ - the plain counterpart of です: 学生だ gakusei da -> 学生じゃない gakusei ja nai -> 学生だった gakusei datta -> 学生じゃなかった gakusei ja nakatta. (じゃない is the everyday form; ではない de wa nai is its written, formal twin.) な-adjectives are identical: 静かだ, 静かじゃない, 静かだった, 静かじゃなかった.

Step 5 - Negative requests: 〜ないでください. Take the ない-form, add でください: 撮る -> 撮らない -> 撮らないでください toranaide kudasai (“please don’t take (photos)”). It is the exact mirror of 〜てください.

Without ください, 〜ないで is a linker meaning “without doing X”: 朝ごはんを食べないで、出かけました (Asagohan o tabenaide, dekakemashita) - “I went out without eating breakfast.” Keep this apart from 〜なくて, which states a cause: 分からなくて、困りました (Wakaranakute, komarimashita) - “I didn’t understand, so I was stuck.”

Step 6 - Where the plain form is obligatory. The working rule is that politeness is marked once, on the final predicate, and what stands before it is plain however formal the sentence is. That is why ×食べますとき is wrong and 食べるとき is right. The structures that genuinely demand a plain form are coming next: とき (when), と思う (I think), こと／の (nominalizers), 名詞修飾 (noun-modifying clauses), でしょう (probably). One group behaves differently and is worth knowing early: the clause-linkers から／ので (reason), たら (if) and が／けれど (but) take the plain form as their default but also accept a polite one, which lifts the register of the whole sentence - 日曜日ですから, 今から行きますので, 終わりましたら, 失礼ですが. Verbs and い-adjectives simply plug their plain form straight in. Only だ misbehaves, and the table below is the map.

Forms & patterns

The four-cell plain paradigm.

Group I: the final kana drops to its -a row, then + ない.

た-form: reuse the て-form table, swap て -> た and で -> だ.

Adjectives, nouns and な-adjectives.

What だ does in front of each dependent (verbs and い-adjectives just attach unchanged).

Examples

Common mistakes

• 食べますとき (tabemasu toki) -> 食べるとき (taberu toki) - politeness is marked once, at the end of the sentence. とき always attaches to a plain form, however formal the sentence is.
• 買あない (kaanai) -> 買わない (kawanai) - verbs ending in う take わ, not あ. Regular for all of them: 会う -> 会わない, 使う -> 使わない.
• お金があらない。 (Okane ga aranai.) -> お金がない。 (Okane ga nai.) - ある has no regular negative; the negative is ない. Only the animate いる is regular (いない).
• 学生だので、安いです。 (Gakusei da node, yasui desu.) -> 学生なので、安いです。 (Gakusei na node, yasui desu.) - before ので, のに and んです the copula だ turns into な. Before から it stays だ.
• この本は高いかった。 (Kono hon wa takaikatta.) -> この本は高かった。 (Kono hon wa takakatta.) - an い-adjective drops its い before かった. There is no ×高いでした either.

Tips

• The た-form costs you nothing new. If you can say 飲んで, you can say 飲んだ; if you can say 書いて, you can say 書いた. One table, two jobs - which is why the て-form was worth drilling.
• Every ない-form is itself an い-adjective. That single fact gives you the past (食べなかった), the linking form (食べなくて) and later the adverbial - all by the 高い rules you already know.
• ないで vs なくて: ないで = “without doing / don’t do it” (食べないで出かけた, 食べないでください); なくて = “because … not” (分からなくて、困りました). English collapses both into “not”, so check which meaning you want before choosing.
• Plain does not mean rude - it means unmarked. Use it in dictionaries, notes, diaries, and inside every subordinate clause of the most formal sentence you will ever write. Whether the whole conversation runs in plain style is a separate choice, covered later.