Usage & register · N5–N3
Plain, polite and honorific Japanese
Keep three separate ideas apart: plain grammar, です/ます politeness, and keigo viewpoint.
In short
Plain forms are used with close relationships and inside many grammatical clauses. です/ます adds standard conversational politeness. Keigo adds respectful/humble choices tied to social relationships and whose action is being described.
How it works
A sentence can contain a plain subordinate clause inside a polite main sentence without becoming rude.
Useful contrast
Learning register is not a ladder where every advanced form is “more correct”. The goal is appropriateness.
Examples
明日行くと思います。
I think I will go tomorrow.
先生はいらっしゃいますか。
Is the teacher here?
Common pitfall
When unsure with a stranger, normal です/ます Japanese is usually safer than forcing advanced honorific vocabulary.