The adjective indeterminable describes something that can't be known for sure, like the indeterminable age of your favorite teacher who has a gray beard but no wrinkles and dresses like his students.
Determine describes calculating something, making a final decision, or just being sure. Something that's indeterminable is none of these. Maybe you need more information — or better math skills. Or, the next action a group will make may be indeterminable because not all the members were at the meeting. As you can tell, things that are indeterminable don't always stay that way. In time, most things can be figured out.
