Hangul (한굴), which is the Korean alphabet system; is largely phonetic.  This means that words are written as they are pronounced.  Well, for the most part they are.  There are some exceptions to the rule.  For example the character resembling ㅅ is pronounced differently depending on where it’s located.  When it’s located on top, it’s pronounced as an English “S” sound.  Words like bosu (버스) which means “bus” or suop (수업) which means “class” have this feature.  When the ㅅ is placed on the bottom, it takes on a “T” sound.  Words like net (넷) which means “four” and bitda (빗다) which means “to comb” have this feature.

When ㅅ is placed both on the top and bottom, you pronounce the ㅅ on top as an “S” and the ㅅ on the bottom as a “T”.  So for example sutja (숫자) which is the word for “number” and set (셋) which is the word for “three” follow this example.

When ㅅ is doubled, it will make an English “sh” sound like in the word “shut”.  Words like shit-da (씻다) which means “to wash” and words like shi (씨) which is the suffix for Mr./Ms. all contain this feature.  When the double ㅅ or ㅆ is on the bottom it will make a ”t” sound like itda (있다).  Itda (있다) can mean “to have or to exist” depending on the sentence.

Hopefully this clears up a lot of the pronunciation troubles that you might be having.  Good luck with your Korean studies!