に: Purpose of Motion (買いに行く)

There is a small, high-frequency construction in Japanese that lets you say why you are going somewhere in a single tidy phrase: attach に to the stem of a verb, then follow it with a verb of motion. 映画を見に行く — "go to see a movie." 買い物に行く — "go shopping." This is the purpose-of-motion に, and once you see how it is built you will use it constantly, because in daily life you are always going somewhere in order to do something.

The construction is narrow but reliable. It only works with motion verbs — 行く (go), 来る (come), 帰る (return home), and a handful of relatives — and it demands a very specific verb form in front of に: the masu-stem (連用形), not the dictionary form. Get those two facts right and the rest follows mechanically.

The structure: [masu-stem] + に + [motion verb]

The pattern is [what you're going to do] + に + [go/come/return]. The first verb is reduced to its masu-stem — the form you get by knocking the -ます off the polite form (見ます → 見, 買います → 買い). The に that follows is a fossilized purpose marker: think of it as "for the purpose of." The motion verb at the end carries all the tense and politeness.

Notice in the third sentence that the destination is marked with へ (海へ, "toward the sea"), while the purpose is marked with に (泳ぎに, "to swim"). Keeping the two roles on different particles here reads more smoothly — but as the next tip shows, doubling up on に is also perfectly natural.

The object goes before the stem

Whatever the purpose verb acts on — its direct object — sits in front of it, with を, exactly as it would in a normal clause. The whole [object を verb-stem に] block then behaves like one unit in front of the motion verb.

That last sentence stacks three particles gracefully: まで (as far as the airport), に (the person met — 会う always takes に for its target), and に (the purpose). Japanese tolerates this because each に is doing distinct grammatical work.

する-nouns can take に directly

A large class of nouns — 買い物 (shopping), 旅行 (travel), 散歩 (a walk), 勉強 (study) — describe activities. With these you have two options. You can build the full stem with し (勉強しに行く), or, more idiomatically, you can drop straight to the noun + に (勉強に行く, 買い物に行く). Both are correct; the bare-noun version is lighter and extremely common.

Why the stem, and not the dictionary form?

This is the point English speakers stumble on, so it deserves a real explanation rather than a rule to memorize. The masu-stem (連用形) is the historic connective form of the verb — its whole job across the grammar is to link to something that follows. It is the same stem that connects to -ます, to -たい (食べたい), to -ながら (見ながら), and to compound verbs (食べ始める). This purpose-に is one more thing that clips onto that connective stem.

The dictionary form, by contrast, is a complete, finite form — it can end a sentence on its own. Putting a finished predicate (見る) directly in front of に and then continuing feels, to a Japanese ear, like jamming two full sentences together with no join. That is why ×見るに行く is ungrammatical: you have to shift 見る into its connective shape, 見, before に can grab it. If you already understand the masu-stem, this construction costs you nothing new — it is just one more attachment to a form you already know.

Purpose-に vs. ために: don't overreach

Japanese has a broader, more explicit way to say "in order to": ために, which takes a full dictionary-form verb (日本語を勉強するために日本へ来た, "I came to Japan in order to study Japanese"). Because ために looks like a clean word-for-word match for English "in order to," beginners reach for it everywhere — including where the light stem+に would be far more natural.

Here is the division of labour. Purpose-に is only for motion verbs and only for the immediate, concrete purpose of that movement — you are physically going, coming, or returning to do something. ために is unrestricted: it works with any main verb (not just motion), and it carries a heavier, more deliberate "for the sake of / for the goal of" nuance.

The second sentence cannot use purpose-に (×健康に走る would mean something else entirely) because 走る here is not "going somewhere to do X" — it is the activity itself, done for the sake of a goal. Keep the two apart: if the main verb is 行く／来る／帰る and you're stating why you're moving, use stem+に; for everything else, or for weightier purposes, use ために.

Tense lives on the motion verb

The purpose stem never inflects — it is frozen. All the grammatical information (past, negative, polite, volitional) is carried by the motion verb at the end.

The stem 会い / 取り stays identical whether the trip is past, future, negative, or polite; only 行きました / 戻らない changes. This is a relief for learners — you conjugate one verb, not two.

Common Mistakes

1. Using the dictionary form instead of the masu-stem. This is the number-one error, and it comes straight from thinking "see" = 見る, so "go to see" = 見るに行く.

2. Forgetting to reduce a godan verb to its -i stem. 泳ぐ is not 泳ぐに; the connective stem is 泳ぎ.

3. Using stem+に with a non-motion main verb. The purpose-に construction requires a verb of motion at the end. If your main verb is 待つ, 勉強する, 練習する — anything that isn't going/coming — you must switch to ために.

4. Marking the object with に instead of を. The thing you're going to see/buy/eat is a direct object and keeps を; the purpose-に comes only on the verb stem.

5. Defaulting to ために for simple errands. Not grammatically wrong, but 日本語を勉強するために図書館に行く sounds heavy and bookish for "going to the library to study." Native speakers say 勉強しに行く. Reserve ために for genuinely purposeful, goal-oriented statements.

Key Takeaways

• Structure: masu-stem + に + motion verb (行く／来る／帰る／戻る／出かける…). The object keeps を; the stem is frozen; tense sits on the motion verb.
• する-nouns can attach に directly: 買い物に行く, 旅行に行く, 勉強しに行く.
• The stem is required because this に clips onto the verb's connective (連用形) form — the same stem behind -ます and -たい.
• Use stem+に only for the concrete purpose of going somewhere; for any non-motion goal, or for a weightier "for the sake of," use ために.