Japanese · Pronunciation Guide
oo-i, three beats — a long o held for two of them, then a short i.
ooi
Listen for the ratio: two beats of o against one of i. That 2:1 shape is the entire word, and it's what distinguishes it from the many two-beat words that would otherwise sound similar. Length here is doing lexical work, not decorative work.
Listen: 多い
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今日は仕事が多いです。
Kyou wa shigoto ga ooi desu.
I have a lot of work today.
Listen: 今日は仕事が多いです。
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今日は仕事が多いです has 仕事 (shi-go-to) before it — three crisp short beats with no length at all. Coming straight after that, the held o of 多い should sound conspicuously long. If it doesn't, you're shortchanging the vowel.
"OY" is what English produces: one syllable, one beat, the whole word collapsed into a diphthong. That's a third of the correct length. Hold the o until it feels overdone, then release into a separate short i. Learners who know the word is short in meaning often make it short in sound too — the two have nothing to do with each other.