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?
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?
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:
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:
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.