10.2 Plain (Casual) Forms

What 'plain form' means

Every Japanese verb has a polite style (the ます-forms you met earlier) and a plain or casual style (also called the dictionary or dictionary-based style). The two carry the same core meaning — のみます and のむ both mean 'drink' — but differ in register and in where they can appear. N5 expects you to recognise all four plain forms, convert between polite and plain, and understand why certain grammar demands the plain form.

The four plain forms

Think of plain forms as a 2×2 grid: non-past vs past, crossed with affirmative vs negative.

Dictionary form

This is the verb as listed in a dictionary — のむ, たべる, する, くる. It is the plain non-past affirmative and the base you count syllables from for other conjugations.

The ない-form (plain negative)

• Group I: change the final -u sound to its -a counterpart, then add ない. のむ→のまない, かく→かかない, まつ→またない, いく→いかない. Watch verbs ending in う: they take わ, not あ — かう→かわない (kawanai), not かあない. This 'う→わ' shift is a favourite exam trap.
• Group II: drop る and add ない: たべる→たべない, みる→みない.
• Group III: する→しない, くる→こない.
• Irregular: the existence verb ある ('there is', for things) has no あらない — its plain negative is simply ない.

The た-form (plain past)

Good news: the た-form uses exactly the same sound rules as the て-form, just swapping て→た and で→だ. So のむ→のんだ, かく→かいた, およぐ→およいだ, はなす→はなした, たべる→たべた, する→した, くる→きた. And the same exception carries over: いく→いった (not いいた).

The なかった-form (plain past negative)

Take the ない-form, drop the final い, and add かった (it inflects like an い-adjective): のまない→のまなかった, たべない→たべなかった, しない→しなかった, こない→こなかった.

Polite ↔ plain mapping

When each style is used

Use the polite ます-forms to end sentences with strangers, teachers, and in most JLPT reading passages. Use the plain forms in three situations that N5 tests directly:

1. Casual speech — among close friends and family: きょう、えいがをみる？ (Watching a movie today?)
2. Before a noun (relative clauses) — a verb that modifies a noun must be plain: たべるひと (a person who eats), きのうかったほん (the book I bought yesterday), あそこにいるひと (the person over there). You can never say たべますひと.
3. Inside embedded clauses — before quotation/thought markers and many conjunctions the verb goes plain: あしたあめがふるとおもいます (I think it will rain tomorrow); いっしょにかえりましょうといいました (he said 'let's go home together').

たら and と — N5 conditionals

Two conditionals at N5 build on plain forms. 〜たら attaches to the plain past and means 'if / when': じかんがあったら、いきます (if I have time, I'll go); うちへかえったら、でんわします (when I get home, I'll call). 〜と attaches to the dictionary form and describes an automatic, natural consequence: はるになると、さくらがさきます (when spring comes, the cherry blossoms bloom); このボタンをおすと、きっぷがでます (if you press this button, a ticket comes out). Use と for reliable cause-and-effect and たら for a one-time 'once X happens'. Recognising which plain form each conditional demands — past for たら, dictionary for と — is exactly the kind of distinction N5 grammar items reward.

Plain forms of adjectives and the copula

Verbs are not the only words with plain forms; い-adjectives, な-adjectives, and nouns have them too, and you need them for casual speech and relative clauses. An い-adjective is already plain in its dictionary shape (たかい, expensive); its plain negative is たかくない, its plain past たかかった, and its plain past negative たかくなかった. The polite です simply attaches afterward. The word いい (good) is irregular: よくない, よかった, よくなかった — never いくない.

な-adjectives and nouns rely on the plain copula だ, which is the casual counterpart of です. So きれいだ / きれいじゃない / きれいだった / きれいじゃなかった, and がくせいだ / がくせいじゃない / がくせいだった / がくせいじゃなかった. In casual speech people often drop だ after an adjective (きょうは さむい？), but they keep it for nouns and before quotation こと such as 〜だとおもいます.

A quick conversion drill

Covering the right-hand column and converting from polite to plain is one of the fastest ways to drill this grid:

Common conversion traps

Four errors recur on the N5 grammar section. First, the う→わ shift for ない-forms: かう→かわない, あう→あわない, つかう→つかわない — never かあない. Second, the existence verb ある has no plain negative あらない; it is simply ない. Third, the いく→いった exception applies to the plain past exactly as it did to the て-form, so the past of いく is いった, not いいた. Fourth, remember that a verb directly in front of a noun must be plain: you say いくひと (a person who goes), never いきますひと.

と versus たら — choosing the right conditional

Because both translate loosely as 'if', learners mix them up. Use と for a dependable, repeatable, automatic result — press-and-it-happens, season-and-it-blooms — and note that the clause after と cannot express a request, invitation, or command. Use たら for a one-time 'once/when X happens, (then) I will…', and it can be followed by a request or plan: じかんが あったら、でんわしてください (if you have time, please call). If the second half contains ください, ましょう, or a wish, たら is almost always the correct answer and と is the trap.

Plain form in casual conversation

Beyond relative and embedded clauses, plain form is the everyday register of friends and family. Casual speech typically drops the question particle か and marks a question with rising intonation, or with a casual の: どこへ いく？ (where are you going?), もう たべた？ (have you eaten already?), これ、たかいの？ (is this expensive?). Statements simply end in the plain form: あした こない (I'm not coming tomorrow). Using this register with strangers sounds blunt, which is why the reading passages stay polite — but the listening section includes casual dialogue, so you must still be able to understand it.

〜とき — 'when' takes plain form

Another pattern that demands plain form is 〜とき ('when / at the time of'): にほんへ いくとき、カメラを かいました (when I went to Japan, I bought a camera). The verb before とき is plain, and its tense sets the timing relative to the main clause — いくとき describes 'on the way there', while いったとき describes 'once I had arrived'. This before-and-after timing inside とき is subtle but genuinely testable at N5, so always check whether the verb in front of とき is the dictionary form or the past form before you answer.