Chinese characters were used for writing, before Hangul.

I do not believe that general Korean people knew how to speak Chinese nor to write. But upper class knew how to read and write letters. This is possible because Chinese characters are Logogram, where a written character represents a word or phrase. To indicate Sun, all they had to write was "太陽". You don't need to know the whole language to write. You see those two characters and you know it means Sun.

As another example, the word "天下", can be translated into "World". I believe that it is pronounced as "ten-cia" or something similar in Chinese. (I don't speak Chinese, but I remember hearing this word in movie Hero.) In Korean, we pronounce this word as "Chun-Ha". So, before Hangul, people said "Chun-Ha" verbally, but wrote "天下". I wouldn't be surprised at all, in old days, if Korean pronunciation was almost like Chinese "ten-cia", and just changed over long time into "chun-ha".

Though, I believe the above was not possible for pure Korean. For example, greetings like "An-nyung-ha-se-yo" could not possibly have matching Chinese. Maybe, they picked random characters by sound, like currently people do in China and Japan to write English words, especially a name.

An old written Korean document example could be Samguk-Sagi.

Note: This is purely based on what I learned and experienced over the years. I do not study languages nor I do have no proof. ;)