I've finished my work.
I finished my work.
When do I use one or the other?
I've finished my work.
I finished my work.
When do I use one or the other?
It's a subtle distinction. The present perfect describes the present ... right now, you can be described as having finished your work. The past describes an action that took place in the past ... five minutes ago, you finished your work.
I've finished makes a more important connection between the action of finishing and the situation the speaker is in.
For example: I've finished my work so we can now go to the park.
When there is a mention of time use the simple past, e.g:
I arrived to the library 20 minutes ago.
But when time is absent from a sentence, use the present perfect, because it's meant to focus on the action, not the time.