Why is kiru represented as kiri in おじいさん が 山 へ 木 を きり に いけば

Question

I can't fathom why the verb kiru is in the masu form without the masu attached in this sentence:

おじいさん が 山{やま} へ 木{き} を きり に いけば

What is the purpose?

Answer 1

You may make sense of this grammar pattern if you know the masu-stem of a verb can act as a noun. As you probably know, に is a particle that usually takes a noun representing a destination, a goal, a resulting status, etc. It roughly corresponds to the English prepositions to, for or into.

So you can use ～に行く with simple nouns and suru-verbs:

• 買い物に行く to go for shopping
• 面接に来る to come to take a job interview

Likewise, when you use ～に行く with a verb representing a purpose, a form that has a noun-like quality is expected, and this is where the masu-stem comes into play:

• 映画を見に行く
• 遊びに出かける

That being said, I think this masu-stem + motion verb construction should be learned by rote. So-called "masu-stem" has various usages, and it may not be always possible to explain it logically. After all, "masu-stem" (or 連用形) was named after its most important function, but its usage is not limited to what the name suggests. See this for details.