に can be a very difficult particle to master in Japanese, as it has many different translations in English. These translations are usually ‘in’, ‘at’, ‘to’, ‘for’, or ‘on’. Let’s look at an example of each meaning.

• 彼（かれ）は部屋（へや）にいますよ。
  He is in his room. (に as ‘in’)
• 私（わたし）は家（いえ）にいます。
  I'm at home. (に as ‘at’)
• 公園（こうえん）に行（い）く。
  To go to the park. (に as ‘to’)
• 私（わたし）は誕生日（たんじょうび）にケーキを食（た）べる。
  I will eat cake for my birthday. (に as ‘for’)
• バスに乗（の）る。
  To ride on the bus. (に as ‘on’)

While these examples may seem quite confusing at first, it will be easier to think of に as always having one ‘basic’ meaning. That meaning is to be the ‘end-point’ of an action. This means that whatever action is being described, can only be considered complete/able to be completed once you’re at/while you’re at the location/time described in the sentence.

• マンションに住（す）む。
  To live in an apartment.
• 旅館（りょかん）に泊（と）まる。
  To stay at a hostel.

Verbs like 住（す）む (to live/dwell) often confuse learners, as they use に instead of で. This is also due to the fact that 住（す）む is a verb that can only be ‘completed’ at the destination. (You can only complete the act of living/dwelling in the place that you live, so to speak).