に (ni)

に has several core uses: direction, time, location of existence, and indirect object (the recipient of an action).

Direction / destination

学校に行きます。
Gakkō ni ikimasu. — “I go to school.”

Time

今日に行きます。
Kyō ni ikimasu. — “I go today.”

Indirect object (to someone)

友達に電話します。
Tomodachi ni denwa shimasu. — “I call my friend.”

When you hand someone a gift, the recipient is often marked with に — see our happy birthday post for a real-world example.