Endings convey grammatical functions that are essential to the context, such as tense, aspects, type of the sentence, conjuctions, speech levels, etc. First there is the Pre-final Endings and the Final Endings. Now I won't be listing down all existing pre-final and final endings existing in the Korean Language, because I still don't memorize all of it. What I'm gonna do here is show you what's the difference between the two, and hopefully, you'll understand how to use them.

Pre-final endings are those inflectional elements between the stem and the final ending. There are a lot of pre-final endings but probably the most commonly used ones are the honorific suffix (으)시 and the past tense marker 었/았 (which will have a separate chapter dedicated of the use of it). Since Korean language is a very heirarchial one, the use of the honorific pre-final ending (으)시 is common. 

Final-endings can be separated into two other types, the sentence and non-sentence final endings.

As the name itself, non-sentence endings are not the endings for a sentence. In English, we have the conjuctives (i.e but, because, although) and that would be the closest thing the non-sentence endings would be. Common clausal conjunctives are:
~고 - and
~어서/아서 - because
~(으)면서 - while
~지만 - although
~은데/는데 - but
~도록 - in order to

Sentence final endings usually indicate the speech level of the sentence. As I have said, Korean language is heirarchial, so it is i