Korean · Pronunciation Guide
ttae. One syllable, and everything is in the opening consonant: a tensed ㄸ made with a locked throat and no breath, over a short flat ㅐ.
ttae
A single brief block. Because 때 is a dependent noun it always follows a modifier, so it forms a rhythmic unit with the clause in front of it and never stands alone with any prominence of its own.
Listen: 때
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시간이 있을 때 전화하세요.
Sigani isseul ttae jeonhwahaseyo.
Call me when you have time.
Listen: 시간이 있을 때 전화하세요.
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시간이 있을 때 전화하세요 puts 때 after the -을 form, and that ㄹ tenses nothing here because 때 already begins with a tensed consonant. The 있을 in front does resyllabify, giving i-sseul with a full double s.
Softening the ㄸ gives 대 and aspirating it gives 태, both real syllables, so this one-syllable word sits right in the middle of a three-way contrast with nothing else to help a listener. Lock the throat, release cleanly, and keep the vowel short and flat.