I think this might help you. http://nihongoichiban.com/home/japanese-grammar-particles/ The answer is not very simple. "NI" is listed in in numbers 4, 7, 15-16, 30-32, 44, and 50 (etc) in the chart linked above. It doesn't have one meaning, it has many. The most common things it does is mark time or location. In both cases it will come after the time or location word. For example: 2-ji ni densha ga kimasu. = The train comes at 2:00 Toukyou ni ikimasu = I go to Tokyo.