に (NI) is used to locate a place (今会議室にいます, Ima kaigijitsu ni imasu, I'm in the meeting room now).
に is frequently used after spatial locators "on, at, in, next to, etc."
Tamago wa reizouko no naka ni arimasu.
The eggs are in the refrigerator.
The particle に (NI) can be used to mark time, and can be used in many situations (hour, date, length, season, etc.).
Gogo sanji han ni eki de aimashou.
Let’s meet at 3:30 PM at the station.
Shigatsu youka ni kekkon shimashita.
I got married on April 8th.
Expresses a movement toward a precise destination. へ (pronounced "E" and not "HE" in that case) is also used, but focuses more on the direction than on the destination.
Gakkou ni ikitakunai.
I don’t want to go to school.
に (NI) can be used after the conjunctive base of a verb to express the goal of an action. The conjunctive base is the one used with polite form -MASU (SHImasu, TABEmasu, etc.). This structure can be translated as "in order to do."
Kimura san wa hon o kari ni toshokan e ikimashita.
Mr. Kimura went to the library to borrow a book.
Designates an action's recipient in structures such as "to give to someone," "to send to someone," "to ask someone," "to teach someone," etc.
Otouto wa inu ni esa o yarimashita.
My younger brother gave food to the dog.
In the structure "noun + に (NI) + verb of movement," the particle に can denote an inward movement. For example, to enter a room, but also in a more abstract way (to participate, to enroll, to meet, etc.).
Shiranai hito ga watashi no ie ni haitte kimashita!
A stranger has entered my house!
Yamada san wa sono purojekuto ni sanka shimashita.
Mr. Yamada took part in this project.
に (NI) is used to indicate the result of a change, mainly with the verb "to become." There is an idea of movement from one state to a new state.
Kyandii o tabete shita ga midoriiro ni natta.
I ate a candy and my tongue became green.
In a passive structure, に (NI) is used to indicate the agent, the grammatical element that is at the origin of the action (by him, by the train, etc.)
Senshuu no nichiyoubi kanojo ni furareta.
I was dumped by my girlfriend last Sunday.
In a causative sentence (to make someone do something), に (NI) is used to indicate the person who is made to do something.
Joushi wa watashi ni zangyou sasemashita.
My superior had me do extra hours.
Be careful, in a passive-causative structure (to be made to do something by someone) it can become complex to identify the right elements correctly, に is used to indicate the agent (the one is at the origin of the action).
Chichi ni shibafu o karasaremashita.
I was made to mow the lawn by my father.
に (NI) introduces the information on which an action is based. It can often be translated as "according to, based on, etc."
Zeikin wa shuunyuu ni motozuite kimerareru.
Taxes are calculated based on the revenue.
に (NI) can be used to make an enumeration of elements usually in a written style. When speaking, it's more common to use the particle と (TO) or や (YA) instead.
Matsuda san wa inu ni neko ni usagi o katte imasu.
Mr. Matsuda has a dog, a cat and a rabbit.
に (NI) is used to indicate the basis for a comparison, the criterion on which the comparison is built.
Jon san wa okaasan ni nite iru.
John looks like his mother.
Yonin ni hitori no hito ga arerugii de kurushinde imasu.
One person out of four is suffering from allergies.
に (NI) is used to emphasize the meaning of a verb when using the following structure: "Verb (conjunctive base) + に + Verb."