What is に ( ni)？

There is a particle that is called に(ni) in Japanese. This is very fundamental parts to make Japanese sentences. However, there are several different usage of it.

友達の家に遊びにいく

I go to my friend's house to play.

In this sentence, に appears as 「遊びに」

In that way, the particle shows the reason why you go to friend's house.

そこにある本をとってください

Please take that book.

In this sentense, そこに contains に. In translation そこに is 'that'. So the particle can be a indicator that things are placed.

In this way, you can extend usage by keeping in mind 「オノマトペ」, which are some kind of words that indicate certain place. For example, これ　それ　あれ.

When you combine オノマトペ with に, オノマトペ should end with Hiragana こ.

一緒に歌いましょう

Let's sing together.

一緒に means together and that is adverb. So particle に is often works to add some additional meanings on verbs. In this usage, you can use

ともに ( together)

別々に ( individually)

意欲的に ( eagerly)

具体的に (concretely)

真面目に ( seriously)

The similar usage as first example

この杖は歩くのには向いていない

This stick isn't good at walking.

歩くのに restrict usage of the stick. In that sense, it shows object to use something, especially in this example usability of the stick.

・indicator

・show object

・adverb

Most of the case に is used three usages above.

外へ走りに行ったが、雨が降ったのでベンチの上に置いてあった傘をさした。唐突に降りはじまた雨は一晩降り続いた。

This sentences contains three different meaning of に. Let's try to differenciatte one by one.