Japanese · Pronunciation Guide
ma-chi-ga-i, four beats — the last two are separate morae, not the diphthong of English "guy."
machigai
The crest falls on ga, and none of the four units outlasts another. The word's difficulty is concentrated at the end, where two adjacent vowels have to stay separate; the first half is straightforward. If your version has three beats, the ga-i fused.
Listen: 間違い
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この番号は間違いです。
Kono bangou wa machigai desu.
This number is wrong.
Listen: この番号は間違いです。
Free browser voice. Hear it read by a real native speaker inside the LingoDrop app.
この番号は間違いです contains 番号 (ba-n-go-o), four morae with a nasal beat and a long o. So the sentence pairs a word that gets its length from a hum and a held vowel with one that gets its length from an extra vowel beat — three different mechanisms in two words.
"ma-chi-GUY" is the near-universal version, and it's wrong in two ways: the stress peak, and the fusion that turns four morae into three. Pull the ga and i apart with a deliberate pulse on each, and level out the emphasis. The chi also needs softening — English ch is heavier and more aspirated than the Japanese affricate.