-ni iku ( Genki I Chapter 7)

verb stem + にいく

If a person moves to another place in order to do something, we can describe their movement and its purpose this way:

destination of movement に/ へ the purpose of movement  に いく/くる/かえる

The purpose of movement can either be a noun or a phrase with a verb, its object and so forth. Verbs describing the purpose of a movement must be in their stem forms.

デパートにかばんをかいにいきました。 I went to a department store to buy a bag.

メアリーさんはにほんににほんごをべんきょうしにきました。

Mary has come to Japan to study Japanese.