These prefixes always remain attached to their infinitives. The inseparable prefixes are unstressed syllables, as compared to the separable prefixes which can stand alone as different words. Some examples of verbs with inseparable prefixes are besuchen - to visit, erzählen - to tell, gewinnen - to win, andversprechen - to promise.
Unter and über can function as separable prefixes, but they are much more commonly used as inseparable prefixes. When prefixes are stressed, they are separable; when they are not stressed, they are inseparable. The stress on the following verbs in not on the prefix, so they are all inseparable: unterhalten - to entertain,unternehmen - to undertake, überholen - to overtake, and übersetzen - to translate.