Particle indicating location or time (at, in, on, during); marks where something exists or when something happens.
学校に行きます。
I go to school.
三時に会いましょう。
Let's meet at three o'clock.
机の上に本がある。
There is a book on the desk.
夏に旅行したい。
I want to travel in the summer.
Marks direction or target (to, toward, into) and the resulting state (become).
右に曲がってください。
Please turn to the right.
彼は医者になった。
He became a doctor.
箱の中に入れてください。
Put it into the box, please.
日本に行く予定です。
I plan to go to Japan.
Indicates purpose (for); used in phrases like 'V‑に行く／来る' to show intent.
買い物に行きます。
I'm going shopping.
日本語を勉強しに来た。
I came to study Japanese.
写真を撮りに公園へ行った。
I went to the park to take pictures.
Shows cause or reason (because of, due to); often seen in formal or written expressions.
台風により電車が止まりました。
Trains stopped due to the typhoon.
病気により欠席しました。
I was absent because of illness.
地震により道路が壊れた。
The road was damaged because of the earthquake.
Marks the agent or source in various constructions (by, from) — for example the giver in 'もらう' or the agent in passive sentences.
友達に本をもらった。
I received a book from a friend.
彼に叱られた。
I was scolded by him.
この手紙は先生に書かれたものだ。
This letter was written by the teacher.
Indicates role or capacity (as, in the role of).
彼は代表に選ばれた。
He was chosen as a representative.
私は通訳に来ました。
I came as an interpreter.
教師にふさわしい態度を取ってください。
Please behave in a manner suitable for a teacher.
Indicates rate, per, or frequency (per, a — e.g. 'once a month', 'per person').
大人一人につき千円です。
It's 1,000 yen per adult.
月に一回歯医者に行く。
I go to the dentist once a month.
人数に応じて席を用意します。
We will prepare seats according to the number of people.
Used to indicate addition or inclusion (in addition to, besides); often appears in colloquial patterns like 'A に B も'.
彼は日本語に英語も話せる。
He can speak English in addition to Japanese.
会議には部長に課長も出席した。
At the meeting, the manager as well as the section chief attended.
この店は野菜に果物も売っている。
This shop sells fruits as well as vegetables.
Archaic usage meaning 'if' or 'although' found in classical/literary Japanese; rarely used in modern speech.
いかにせむに、人の心は変わるものだ。
Whatever one does, people's hearts will change. (literary/archaic)