"Ι could" translates to "μπορούσα", past form of "μπορώ". (Or "θα μπορούσα", for polite requests).
Μπορούσα να πάω, αλλά δεν πήγα - I could go, but I didn't. (unlike the english, we don't say "I could go, but I won't" in greek. They might be interchangeable, but you almost never encounter it in spoken Greek. Not is such cases.)
Μπορώ να πάω, αλλά δεν θα πάω - I can go, but I won't.
Also in politeness cases (where could and can are interchangeable):
Θα μπορούσες/Μπορείς να μου φέρεις ένα ποτήρι νερό; - Could/Can you bring me a glass of water?
I hope I helped ^.^