If 옷 is pronounced as if there is a "t" on the end, then why isn't the ㅅ pronounced as "t" when saying 옷을 입다?

質問を翻訳

Ok.. it would be difficult. It is about when the letter that starts its pronounciation with vowels comes after the letter that has final consonant.

Korean(language) can be divided for Empty Morpheme and Full Morpheme. If the empty morpheme(which pronounciation starts with vowels) comes after the noun(like '옷' + '-을'), soft mutation occurs. So, the last consonant of the noun becomes the first consonant of the empty morpheme. (옷을 -> [오슬])

However, if the full morpheme(which starts its pronounciation with vowels) comes after, it doesn't happen.
(ex. In '옷 안', '옷' and '안' are both full morpheme. So, if each pronounciation process happens, it is pronounced like [옫 안]. But, because Korean basically has soft mutation at pronounciation, it is finally pronounced [오단])

It is veeeery difficult and deep part of Korean grammar/pronouncation rules that are taught in high-school. If you want to search for, I recommand these words: 연음, 연음법칙, 음절의 끝소리 규칙, 실질형태소와 형식형태소