Two Forms of the Same Language
Japanese verbs have a plain form and a polite form, and the choice is social rather than grammatical. The polite form (です・ます) is the safe default; the plain form is what friends and family actually use.
The information is identical. The difference is who you are talking to.
The Plain Forms
Notice that い-adjectives look identical in both registers — only the です disappears. That is why polite い-adjective sentences are so easy to form.
Plain form is not rude
It is the normal, neutral register between people of equal standing who know each other. It becomes rude only when used upward — to a teacher, a boss, a customer or a stranger.
When to Use Which
The switching point
In real conversation you often start polite and drift to plain as the relationship warms — but the senior person usually initiates the switch. Until then, stay polite. Remaining polite is never an error; becoming casual too early is.
Polite Outside, Plain Inside
This is the pattern that surprises learners. When a plain clause is embedded inside a polite sentence, the embedding clause stays polite while the inner content goes plain.
The main verb (思います, 言いました) carries the politeness; the quoted content is plain. Mixing 降りますと思います sounds wrong to a Japanese ear.
Same rule for から and ので
In 雨が降るので、行きません the reason clause is plain and the main clause is polite. Politeness attaches once, at the end.Casual Speech Contracts
Spoken casual Japanese shortens forms in predictable ways. Recognition matters more than production at first.
Casual speech also adds sentence-final particles that carry tone: よ (informing), ね (agreeing), な (muttering to oneself), じゃん (isn't it), し (listing reasons).
Common Mistakes
There Is a Third Level
Beyond plain and polite sits honorific language — 尊敬語 for the other person's actions and 謙譲語 for your own. It is a separate system, mostly needed for work and service situations, and it is covered in the Japanese keigo guide.
For exams and daily life, the plain/polite split is what matters first. Practise by rewriting the same five sentences in both registers, then check them with the Lingaso writing analyser for consistency.
Practice
Convert each sentence to the other register.
- 今日は忙しいです。 → plain
- 映画を見た。 → polite
- その店は安くないです。 → plain
- 学生だった。 → polite
- 彼は来ないと思います。 → casual
FAQ
Is it rude to use plain form in Japan?
Only when used upward. Between friends, family and peers of equal standing, plain form is the normal register. Using it with teachers, bosses, customers or strangers comes across as blunt or presumptuous.
Should I learn plain or polite form first?
Learn the polite form first, because it is the safe default and it is what textbooks and beginner courses use. Add the plain forms early, however — you cannot read casual text or follow natural conversation without them.
Why do sentences use plain form inside polite sentences?
Because politeness is marked once, on the main clause. Quoted content, reason clauses and embedded statements stay plain: 雨が降ると思います, not 降りますと思います.
What is the difference between です and だ?
They are the same copula in different registers: です is polite, だ is plain. だ cannot follow an い-adjective, which is why 高い stays 高い in both registers.