Japanese Plain Form: Dictionary, Nai, Ta, and Nakatta

What plain form actually is

Plain form, sometimes called the dictionary style or futsūkei, is the base shape of a Japanese predicate without the politeness layer that desu and masu add. Every verb has four of them: the dictionary form for non-past affirmative, the nai form for non-past negative, the ta form for past affirmative, and the nakatta form for past negative. Nomu, nomanai, nonda, and nomanakatta are one verb wearing four grammatical outfits, not four separate words to memorise.

The reason to build them early is structural. Japanese attaches grammar to the end of a plain predicate, then adds politeness only once, at the very end of the whole sentence. So a sentence can be perfectly polite while containing three plain forms inside it. If you only know masu forms, you can make simple statements but you cannot quote a thought, describe a noun with a clause, or state a reason, because those patterns have nowhere to attach.

Building plain form from the masu form you already know

Most learners start from masu forms, so the fastest route to plain form runs through them. Drop masu to get the stem. If the stem ends in an e sound, or is a short i-sound stem such as mi or i, you are almost always looking at an ichidan verb: add ru. Tabemasu becomes taberu, mimasu becomes miru. If the stem ends in an i sound after a consonant, it is a godan verb: move that final i sound one row down to the matching u sound. Nomimasu has the stem nomi, so nomi becomes nomu; kakimasu becomes kaku; hanashimasu becomes hanasu, because the shi row maps to su.

The two irregulars are simply learned. Shimasu becomes suru and kimasu becomes kuru, written 来る. Watch for godan verbs that look ichidan: kaerimasu is kaeru, hairimasu is hairu, and hashirimasu is hashiru, all godan despite the ru ending. Because their masu stems end in ri rather than an e sound, the masu-form route quietly gets these right, which is one reason it is a safer starting point than guessing from the dictionary form alone.

The nai form and the u-to-a shift

For a godan verb, replace the final u sound with the matching a sound and add nai: kaku becomes kakanai, nomu becomes nomanai, matsu becomes matanai because the tsu row's a sound is ta, and hanasu becomes hanasanai. The one wrinkle is verbs ending in the vowel u, such as kau and iu. Their negatives are kawanai and iwanai, with a w rather than a bare a. This is not an exception invented for learners; the w survives from the older shape of those verbs.

Ichidan verbs are easy: drop ru and add nai, so taberu becomes tabenai and miru becomes minai. Suru becomes shinai. Kuru becomes konai, written 来ない, and the reading changes from ki to ko, which is worth saying aloud a few times because the kanji hides it. Finally, aru is irregular: its plain negative is not aranai but simply nai.

The ta form and the te-form shortcut

If you know the te form, the past is nearly free: keep the same sound change and swap the ending from te to ta, or de to da. Nonde becomes nonda, kaite becomes kaita, itte becomes itta, tabete becomes tabeta, shite becomes shita, kite becomes kita. This works for every verb including the irregulars, which is why teachers usually teach the te form first and hand you the past tense as a bonus.

The nakatta form is the past negative, and it is built from the nai form rather than from the verb directly. Take the nai form, drop the final i, and add katta: nomanai becomes nomanakatta, tabenai becomes tabenakatta, konai becomes konakatta, nai becomes nakatta. This is the same katta ending that i-adjectives use, which is a useful hint that nai behaves like an i-adjective throughout Japanese grammar.

The full plain-form table by verb class

Read the table down a single column rather than across a single row. Reading down the nai column trains the u-to-a shift as one motion, and reading down the ta column exposes the godan sound groups that the past tense guide covers in detail. Rows across are useful only once each column is automatic.

Plain forms of adjectives and nouns

Verbs are only half of it, because Japanese sentences can end in an adjective or a noun instead. An i-adjective already is its own plain form: takai is the plain non-past. The negative is takakunai, the past is takakatta, and the past negative is takakunakatta. Notice that desu adds politeness without changing any of this, so takakatta desu is correct and takai deshita is not.

Na-adjectives and nouns behave alike because both rely on the copula. The plain non-past is da, the negative is ja nai, the past is datta, and the past negative is ja nakatta. In writing and in careful speech, de wa nai and de wa nakatta replace the contracted ja forms. One trap: before to omou and node, a na-adjective or noun keeps na or a form of the copula, so gakusei da to omoimasu and shizuka na node are correct while gakusei to omoimasu is not.

The adjective guide on this blog covers comparison and modification in more depth. What matters here is that the four-way past and negative grid is identical in shape across all four word types, so one drill covers everything you can put at the end of a Japanese clause.

Where plain form is required regardless of politeness

This is the part that turns plain form from a register topic into a grammar necessity. When a pattern attaches to a predicate, it usually attaches to the plain one, and the politeness is added later at the very end of the sentence. Ashita ame ga furu to omoimasu is polite as a whole, yet furu inside it is plain, because to omou never takes a masu form before it. The same holds for relative clauses: kinō katta hon desu is polite, but the clause describing the book is plain.

Two of these deserve a caveat. Kara and node both give reasons, and both accept polite forms before them in formal delivery, such as an announcement or a careful business explanation. In ordinary speech, however, the plain form before kara or node is the norm even when the sentence ends in desu or masu. Node in particular sounds softer with a plain form in front of it, which is why it is the reason marker of choice for customer-facing apologies.

Polite sentences full of plain forms

Practise saying these aloud and notice the shape: plain in the middle, polite at the end. Once that pattern feels normal you stop trying to insert masu forms in the middle of clauses, which is the single most common structural mistake at this stage.

When a plain ending changes the register

Now the other half. Putting a plain form at the very end of a spoken sentence signals closeness or informality, and that choice is separate from everything above. With friends, family, and classmates of the same age, plain endings are the default and masu endings can sound oddly stiff. Toward a customer, a manager, or someone you have just met, keep desu and masu. The safest habit is to let the other person set the register and follow it.

Two details make plain endings sound natural rather than blunt. First, da is often dropped after a noun or na-adjective in casual speech, especially by women: kirei rather than kirei da. Second, casual questions rise in pitch without ka, so iku with a rising tone means are you going. Adding ka to a plain form makes a sentence sound abrupt or masculine, so avoid iku ka until you have heard it used in context. The polite-versus-casual guide on this blog goes further into the social side.

A drill that makes the switch automatic

• Pick five verbs a week, one from each godan sound group, and chant all four plain forms in the order dictionary, nai, ta, nakatta.
• Immediately convert each into a polite sentence with a plain form buried inside, such as tabeta ato de renraku shimasu.
• Take a masu sentence you already say and rebuild it three ways: with to omoimasu, with node, and as a relative clause.
• Record yourself telling one thing you did yesterday in plain endings, then retell the same story in masu endings without pausing.
• Write your daily notes in plain form with da or de aru, since that is where plain endings are genuinely standard in writing.
• Whenever a correction shows a wrong middle-of-sentence form, say the fixed sentence three times before moving on.