The word queō ("I am able") is back-formed from nequeō ("I am not able") and, to the best of my knowledge, is equivalent in meaning to possum.
When would one use queō over possum, or vice versa?
Its hard to establish an exact translation, but I believe that possum is something closer to "I can", rather them "I`m able to". Considering other romance languages that have a translation of "I can" as: posso (italian), posso (portuguese), je peux (french) and puedo (spanish); the similarity is clear. About its use, is just like in English.