Japanese · Pronunciation Guide
tsu-yo-i, three beats — a word-initial affricate, then two more morae that mustn't fuse.
tsuyoi
Crest on yo, three units, and no difference in length between them. The word has a hard onset and a delicate ending, and English speakers tend to over-invest in the tsu and then rush the final i — keep the tempo even so all three beats get the same time.
Listen: 強い
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彼はとても強いです。
Kare wa totemo tsuyoi desu.
He is very strong.
Listen: 彼はとても強いです。
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彼はとても強いです pairs it with とても, three quick short beats that set a tempo. Then 強いです adds three more plus the devoiced copula. There's nothing long in the sentence at all, which makes it a pure exercise in keeping short morae from getting shorter as you go.
"tsoo-YOY" is the usual output: a rounded u, a stressed middle, and the last two morae fused into the vowel of English "boy." Spread the lips, level the emphasis, and give the closing i its own pulse. The tsu also loses its t in many learners' mouths — make sure the stop is audible inside the single affricate gesture.