買う
かう
kau
Conjugation
= to buy
| Form | Japanese | Romaji | Meaning |
|---|---|---|---|
| Dictionary form The plain form you find in a dictionary |
買う かう |
kau | to buy |
| Masu form Polite present / future |
買います かいます |
kaimasu | buy (polite) |
| Te form Connects clauses, makes requests |
買って かって |
katte | buy (connective) |
| Ta form Plain past |
買った かった |
katta | bought |
| Nai form Plain negative |
買わない かわない |
kawanai | not buy |
| Masen form Polite negative |
買いません かいません |
kaimasen | not buy (polite) |
| Nakatta form Plain past negative |
買わなかった かわなかった |
kawanakatta | did not buy |
| Potential form Can / be able to |
買える かえる |
kaeru | can buy |
| Volitional form Let’s / I think I’ll |
買おう かおう |
kaou | let’s buy |
Kau (買う) is a clean example of how a godan verb changes shape across its forms. The shift you see in the negative stem, where う becomes わ rather than あ, is the regular rule for う-ending godan verbs, which makes kau a reliable model for checking your own conjugations of that group.
You can tell kau is godan (う-verb) rather than ichidan (る-verb) because its dictionary form ends in う, not る. The negative form confirms it: godan verbs shift the final う-row sound across the five vowel rows of the same consonant column — あ, い, う, え, お — as they conjugate, while an ichidan verb like 食べる (taberu) simply drops る and adds ない, giving 食べない (tabenai) with no such shift. For kau, instead of the あ-row sound, う-final verbs take わ — consistently, for every verb in the group. So the negative stem is kawa-, not kaa-, giving 買わない (kawanai) instead of the wrong shape 買あない. Every other う-ending godan verb (言う, 会う, 使う, and so on) follows this identical わ-shift, so once kau is solid the rest of that group falls into place.
Kaimasu is the form for speaking to someone you are not close to, such as a shop clerk or a coworker, or for writing where politeness is expected. Katte is the connective shape you reach for mid-sentence, before another verb or before ください to make a request. Katta reports a finished purchase in plain speech, the register used with friends or in a diary. Kawanai and its past kawanakatta state a decision not to buy, in the same plain register as katta, while kaimasen is the polite version of that same refusal. Kaeru expresses ability or possibility — that buying something is within reach, whether financially or in terms of availability. Kaou is what you say to propose the purchase, to yourself or to someone with you.
Everyday use
駅前のパン屋でパンを買った。
Ekimae no pan’ya de pan o katta.
I bought bread at the bakery in front of the station.
Casual / Social Media
セールになるまで買わない。
Seeru ni naru made kawanai.
I’m not buying it until it goes on sale.
Formal / Cultural context
こちらの商品はオンラインストアでも買えます。
Kochira no shouhin wa onrain sutoa demo kaemasu.
This item can also be bought through our online store.
One slip is putting あ where わ belongs and writing 買あない, but the correct shape is 買わない (kawanai), with わ standing in for that missing あ-row sound. A separate trap sits in the potential form: 買える (kaeru) shares its romaji with two unrelated verbs, 帰る (kaeru, to go home) and 変える (kaeru, to change) — the kanji and meaning are completely different in each case, so context alone tells you which one a sentence means. Learners also sometimes stretch out the small っ in 買って (katte) as if it were a long vowel; that っ is a full beat of silence before the て, not a lengthening of the a sound.