There are two different ways to express what you have done: hef (“have”) and búinn (“done/finished”).
You use “búinn” when you’re speaking of a recent or a specific incident, while you use “hef” if you’re speaking more generally. There are many instances where either can be used, but when in doubt you should use “ég er búinn” instead of “ég hef”.
- „Ég er búinn að borða kjúkling“ (I have already eaten chicken) – This is speaking of a recent or a specific incident, such as explaining that you’ve eaten chicken today.
- „Ég hef borðað kjúkling“ (I have eaten chicken) – This is speaking more generally, such as explaining that you have had chicken some time in your life.