As many languages, Korean spelling is sensitive to the integrity of words or part of words. In 감사합니다, 감사 is a word, and both 감 and 사 are meaningful syllables (corresponding to Chinese characters). The 하- part is also a word, a verb meaning "do", so that if you split the ㅎ "h" and the ㅏ "a", the integrity of the root is lost, right? Same holds for 다, which you can find as an ending in many contexts. It wouldnt make much sense having *합닏-아 on the one hand and 한-다 on the other, because it s the same ending in both cases. Now there are rules of composition that forbid to have some sequences of letters within the same graphic syllable (for instance ㄴㄷ), because those sequences cannot be pronounced as one unit at the beginning or the end of a single syllable. This may explain some accomodations with the integrity principle... The-s ame princip le-app ly to-oth er-lang uages-as well... ;-)

I've just checked your profile and now I understand why it is an issue for you. Swedish is quite a bit phonetic, right? Anyway...I'm afraid there's no such rule about it. If you were asking why Korean spelling has been established like that, that's beyond me. Though Korean is much more consistent in spelling and pronunciation than English as far as I can see. You'll just have to memorize words by heart. One thing you need to know - 감사합니다 cannot be 감샇압니다 since ㅎ is always silent when followed by a vowel. Some Korean people use 낳아요 to mean 나아요 like some English speakers use it's to mean "its".

I am not sure if this will help clarify your question. But as Gloups has explained, its to do with the integrity of the word. These two video on Hangeul will explain. http://www.talktomeinkorean.com/category/lessons/learn-hangeul/

i can say its just like a rule also. in engilsh we have the same like that rule. example."doubt"--- how u read this one? its one syllable, you cannot read this into two right? and also we have many silent letter. in hangul--- they have a rule, every consonant attached to the vowel you have to form also this into the one syllable. 감샇압닏아 ---to read this it is gam-sah-ab-nid-a. so this is wrong. 김사합니다---- to read this it is gam-sa-hab-ni-da. so this is the right to pronounce. sometimes we have to memories those spellings because even in english we also memories those words.