I thought I had understood (so far) that when there is no article at all, as here, then it was unspecified between definite and indefinite, and therefore either "a bear" or "the bear" might be understood. And that ten/ta/to was fairly interchangeable between that/the... That has seemed to be the rule so far, but it rejected my answer "The bear is chasing us", and said the correct answer could only be "A bear is chasing us". Ten medvěd could be "the bear" or "that bear. Medvěd alone could be "a bear" or "the bear". Am I getting that wrong?
"The bear" can be used here in only in very specific circumstances. It can be quite confusing to allow it, because people will think it does not matter.
But we have been adding translations like these and I will add it (reluctantly). But be aware it may become removed again.