Confused about simple vowels

I am now just starting the simple vowels section. I will use the 1st vowel as an example. "ㅏ" is how it is shown , as a single vowel able to male the sound "ah". When it comes to the examples it is "아버지" what is this circular character placed in front of the "ah". Have i missed something important or is it further along and I have yet to learn. All help appreciated, great program so far ! Another example 오 for ㅗ what is the circular character.

Well, the circle in front of the vowel helps the vowel make a sound. The circle is silent when it comes before a vowel. You cannot make the vowel make a sound if its just "ㅏ" but if you add the circle before "ㅏ" people can read it as "ah". Do you understand now? If you need further information check out this video: http://www.youtube.com/watch?v=AqFGHMxWtYk