Grammar

Japanese Plain Form: Uses in Clauses, Quotes, and Casual Speech

普N5–N4

The plain form (普通体, futsūtai) is the dictionary form of verbs, plain adjective forms, and だ for nouns/な-adjectives. It is used in casual speech, embedded clauses (〜と思う、〜と言う), conjunctions (〜から、〜ので), and as the base for many N4 grammar patterns.

Plain form vs polite form: the two registers

Every Japanese verb and adjective has two registers: plain (普通体) and polite (丁寧体). Polite forms end in 〜ます/〜です. Plain forms are the dictionary forms. In formal speech, use polite forms. In casual conversation, in embedded clauses, and in writing, plain forms are used.

食べます（丁寧体）→ 食べる（普通体）たべます → たべるto eat: polite → plain

行きません → 行かないいきません → いかないwill not go: polite → plain negative

食べました → 食べたたべました → たべたate: polite past → plain past

Plain form in embedded clauses (〜と思う、〜と言う)

When embedding a clause inside と思う (I think that…) or と言う (say that…), the embedded clause must be in plain form, even in otherwise polite speech. This is one of the most important N4 grammar rules.

明日雨が降ると思います。あしたあめがふるとおもいます。I think it will rain tomorrow. (降る = plain form inside the clause)

彼は来ないと言いました。かれはこないといいました。He said he was not coming. (来ない = plain negative inside と言う)

試験に合格したと聞いた。しけんにごうかくしたときいた。I heard that you passed the exam. (した = plain past)

Plain form with 〜から and 〜ので (reason)

Both から (because) and ので (because/since — more polite) can attach to plain forms when the reason clause is not the final element. ので with plain form sounds slightly rough — many speakers insert だ before ので: 忙しいので → 忙しいので (acceptable) vs 忙しいだから (colloquial).

忙しいから、行けない。いそがしいから、いけない。Because I am busy, I cannot go. (plain form + から in casual speech)

雨が降ったので、中止になりました。あめがふったので、ちゅうしになりました。Because it rained, it was cancelled. (plain past + ので in formal)

学生なので、お金がない。がくせいなので、おかねがない。Since I am a student, I have no money. (な-adjective / noun: だ→な before ので)

Plain form in casual conversation

In casual speech between friends and family, all sentence-final verbs drop to plain form. です/ます endings sound unnatural or overly formal in close relationships. Adding sentence-final particles (よ、ね、な) adds nuance.

明日、どこ行くの？あした、どこいくの？Where are you going tomorrow? (plain + の = casual question)

もう食べた？もうたべた？Did you already eat? (plain past = casual question)

そっか、分かった。そっか、わかった。I see, got it. (plain = casual affirmation)

Common mistakes

Wrong

行きますと思います

Right

行くと思います

Inside と思う, the embedded clause must be plain form (行く), never polite form (行きます). The final verb (思います) carries the politeness.

Wrong

学生だので (before ので)

Right

学生なので

Before ので, the copula だ changes to な: 学生なので. This is a special rule — だ + ので → なので. In contrast, だ + から → だから is fine.

Wrong

Using です/ます with close friends in every sentence

Right

Switch to plain form in casual speech

Consistently using です/ます with close friends or classmates creates distance and sounds formal/stiff. Plain form is the natural register for informal relationships.

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Frequently asked questions

What is the plain form in Japanese?

The plain form (普通体 / futsūtai) is the unconjugated dictionary form of verbs (行く、食べる), the plain form of adjectives (寒い、便利だ), and the copula だ. It contrasts with the polite form (丁寧体) which uses 〜ます/〜です. Plain forms are used in casual speech, embedded clauses, and many grammar patterns.

Why must the verb before と思う be in plain form?

と thought/said introduces a quoted or embedded clause, and all embedded clauses in Japanese use the plain form regardless of the overall formality level of the sentence. The politeness is carried by the final verb: 行くと思います (polite overall, but 行く inside is plain). Using 行きますと思います is a common learner error.

What is the difference between plain form だから and なので?

Both mean "because" but differ in formality and attachment rules. だから: plain/casual, attaches as だ + から → だから after nouns/な-adjectives. ので: more polite and explanatory, but after nouns/な-adjectives, だ changes to な before ので: 学生なので. In polite/formal speech, ので is preferred; だから sounds assertive or argumentative in formal contexts.