The difference is in the numerus. For simplicity, I will drop in the example sentences the adverb gerne, as then we are dealing with the same verbs in German (spielen) and English (to play).
Singular
- Nina spielt Fußball.
→ Nina plays football.
Here, Nina is one person. Therefore, the third-person (not second-person, as you wrote) singular forms must be used (spielt = plays).
Plural
- Nina und Amy spielen Fußball.
→ Nina and Amy play football.
Here, Nina and Amy are more than one person. Therefore, the third-person plural forms must be used (spielen = like).