Japanese · Pronunciation Guide
ka-u, two separate vowel beats — not the single "cow" sound English wants to make of it.
kau
Two morae with the accent on the first, so ka is high and u falls. Two adjacent vowels is the configuration English handles worst, since English diphthongs are single units by definition. Japanese counts them separately, and this verb is where beginners first meet the problem head-on.
Listen: 買う
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パンを買いに行きます。
Pan o kai ni ikimasu.
I'm going to buy bread.
Listen: パンを買いに行きます。
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パンを買いに行きます conjugates to 買い (ka-i), so the second vowel changes but the two-beat structure survives. パン at the head is two morae with a nasal beat. Then 行きます closes with a ki that can devoice. Short as it is, the sentence exercises vowel separation, a moraic nasal and devoicing in turn.
"cow" is exactly what comes out — one syllable, one beat, a rounded glide at the end. Japanese has neither the rounding nor the fusion. Keep the lips flat for the u, hold it long enough to hear it as a vowel, and make sure the word takes two counts. If it rhymes with an English word, something has gone wrong.