Japanese · Pronunciation Guide
ta-i-se-tsu, four beats — the ta and i are separate, and the tsu is one affricate.
taisetsu
Four units of the same length, with the crest on se. Two of those four beats are things English mishandles — a bare vowel that gets absorbed and an affricate whose final vowel gets either split off or over-voiced — so hitting four honest counts here is a real test.
Listen: 大切
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今日は大切な日です。
Kyou wa taisetsu na hi desu.
It's an important day.
Listen: 今日は大切な日です。
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今日は大切な日です ends with 日 read as "hi" — a single mora with a palatal h, quite different from the same kanji read as "nichi" or "ka" in other words. Between 大切 and 日です, the sentence closes on three consecutive short beats with a devoiced su at the very end.
"TIE-set-soo" is the composite error: the first two morae fused into an English diphthong, a stressed second syllable, a split affricate and a rounded final vowel. Pull the a and i apart, level the beats, keep the tsu as one gesture, and let its vowel go voiceless with the lips flat.