When it says you achieved, for example, 80% of the translation (or however it's worded) is there a way to see the correct translation so you can compare and see what you did wrong?
Yes. Simply click the "Current best translation" tab after submitting your translation. Optionally, you can click the button with the two arrows to the right of the original text to see the current best translation without submitting your own (although you won't get any skill points for doing so, so I discourage this).
It just means that your translation is in 80% agreement with all the others submitted. It doesn't mean you've done anything wrong, in fact yours could be better than all the ones put in before you. Sometimes if you are the first person to translate something, it will compare your work to Duobot's. His is almost always very bad, but since yours doesn't agree with his, yours will be scored low. Over time, as other translators rate your stuff, you might find yours will become the highest rated translation.