Beginners learn ます-form first, which is sensible: it is polite and safe. Then they discover that a great deal of grammar attaches to the plain form, and that dictionaries list verbs that way.
Plain form is not slang. It is the base of the language.
What it looks like
| Polite | Plain |
|---|---|
| たべます | たべる |
| のみます | のむ |
| いきます | いく |
| たべません | たべない |
| たべました | たべた |
| たべませんでした | たべなかった |
For adjectives and nouns: たかいです → たかい; しずかです → しずかだ; 学生です → 学生だ.
How to build it
From the dictionary form, which depends on the verb group - so verb groups have to be clear first: Japanese verb groups.
- Group 2 (る-verbs): drop ます, add る. たべます → たべる
- Group 1 (う-verbs): change the い-row sound to the う-row. のみます → のむ, いきます → いく, かいます → かう
- Irregular: します → する, きます → くる
The negative plain form (ない-form) and the past plain form (た-form) have their own guides: the nai-form and the ta-form.
Why you cannot avoid it
Because the grammar requires it. These all attach to plain form:
- ～とおもいます - I think that
- ～といいます - it says / they say
- ～ことができます - can
- ～つもりです - I intend to
- ～まえに / ～あとで - before / after
- ～たら / ～と / ～なら - conditionals
- ～かもしれません - might
- ～ようです / ～そうです - it seems / I hear
Note the politeness: 行くと思います is a polite sentence with a plain-form verb inside it. The politeness lives at the end of the sentence, not in every verb.
That single insight resolves most of the confusion beginners have about when plain form is acceptable.
When to speak in plain form
- With friends of the same age or younger, and with family.
- With colleagues you are close to, after they use it with you.
When not to:
- With customers, managers, or anyone you have just met.
- In any workplace setting until you have heard others do it with you.
If in doubt, stay polite. Nobody is offended by ます-form. The same cannot be said in reverse - see keigo basics for work.
How to learn it
Convert as you go. Every time you learn a verb in ます-form, say the plain form aloud immediately. After a few weeks the conversion stops being a calculation.
Then read: plain form is what written Japanese uses, so any amount of reading practice drills it automatically. Learning kanji and reading go together here.