For now, we will focus on the two most common styles of verbs you will use, polite and casual. As you can see above, there are also two different endings for both polite and casual speech. The way you decide which ending to choose is based on the verb root. If the last vowel in the verb root is ㅗ or ㅏ then you choose the 아요 (polite) or 아 (casual) ending. If the last vowel in the verb root is anything other than ㅗ or ㅏ, then you choose the 어요 (polite) or 어 (casual) ending. Look at the following chart of commonly used verbs and compare the verb root to the ending to get a better understanding of this. I will refer to the Formal ending section next.

English	Verb Root	Dictionary Form	Polite Ending	Casual Ending	Formal Ending
To Have	있	있다	있어요	있어	있습니다
To Be Good	좋	좋다	좋아요	좋아	좋습니다
To Go	가	가다	가요	가	갑니다
To Not Have	없	없다	없어요	없어	없습니다
To Do	하	하다	해요	해	합니다

Let's look at a couple. 있다 means "to have." 있 is the verb root. The last vowel in 있 is ㅣ. Since this is not ㅗ or ㅏ then we know to choose the 어요/어 endings depending on whether we will need to be polite or if it is casual speech.

없다 (~업다) has a verb root of 없. The final vowel in this root is ㅓ, so we need to choose the 어요/어 endings.

If you look at 좋다, this has a verb root of 좋 with a final vowel of ㅗ. So, since that is ㅗ or ㅏ, it must take the 아요/아 ending. Understand the general pattern?