The verb isseoyo (있어요), mentioned in the last lesson, has the particle yo, which indicates politeness.

When adding yo to the stem of a verb, you either add -yo or -eoyo, depending on what consonant (if any) precedes it.

If the last letter is the vowel a or o, you add ayo.
If the last letter is any other vowel, you add eoyo.
If the last letter is a consonant, you add eoyo (like iss-eoyo).