It's past participle:

I swim.
I swam.
I have swum.

I drink.
I drank.
I had drunk.

I eat.
I ate.
I have eaten.

I choose.
I chose.
I have chosen.

I go.
I went.
I had gone.

Here's a weird one:
I come.
I came.
I have come.

Some say the irregular past participles are disappearing. Could be.