A2
The plain form
The Japanese plain forms — dictionary, ない, た and なかった — how the two verb groups conjugate, and why nothing beyond the basics can be built without them.
Everything so far has ended in ます. That is the polite layer. Underneath it every verb has a plain form, and it is the plain form that grammar attaches to — every pattern from here on needs it.
食べる。
Taberu.
I eat.
明日行く。
Ashita iku.
I’m going tomorrow.
Same meaning as 食べます and 行きます, said to someone you are close to.
The four forms
| 食べる | 食べない | 食べた | 食べなかった |
| 飲む | 飲まない | 飲んだ | 飲まなかった |
| する | しない | した | しなかった |
| 来る | 来ない | 来た | 来なかった |
The negative ends in ない and the past ends in た. Both then conjugate further — ない behaves exactly like an い-adjective, which is why the negative past is なかった.
Two groups
Verbs ending in る with an e or i before it usually just drop る: 食べる → 食べない. Everything else changes the vowel: 飲む → 飲まない, 行く → 行かない, 話す → 話さない.
The past follows the same sound changes as the て form — 飲んで and 飲んだ, 書いて and 書いた. Learning one gives you the other.
Where it is not a choice
Inside a sentence, the plain form is the only option. Politeness is carried once, by the final verb, so everything embedded stays plain regardless of who you are speaking to.
行くと言いました。
Iku to iimashita.
He said he’s going.
行きますと言いました。
行くと言いました。
The quoted verb is inside the sentence, so it takes the plain form even in polite speech. Only 言いました at the end carries the politeness.
This is why the plain form cannot be postponed as “casual Japanese for later”. A polite sentence of any length contains it.