Korean · Pronunciation Guide
sa-shil. The second ㅅ palatalizes because of the ㅣ behind it, so two identical letters end up sounding different.
sasil
There is no syllable to lean on. What makes the word worth studying is the pair of ㅅ letters doing two different jobs -- plain in the first block, palatalized in the second -- purely because of the vowels that follow them.
Listen: 사실
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그건 사실이 아니에요.
Geugeon sasiri anieyo.
That's not true.
Listen: 그건 사실이 아니에요.
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그건 사실이 아니에요 attaches the subject particle and lets the ㄹ tap forward, giving sa-shi-ri. The contraction 그건 at the front is worth noticing too: it is 그것은 squeezed down into two syllables.
Saying 'sa-sil' with two identical s sounds is the standard error and it misses the rule entirely -- the second one is 'sh'. Beginners also finish on an English dark l, when the Korean ㄹ needs the tongue tip up and the back of the tongue flat.