A clause with ut or ne and the subjunctive can be the subject or object of a verb, or explain a noun: "it happens that …", "take care that …", "you must …".
It completes a verb that needs a "that" clause: verbs of asking, urging, ordering and taking care (rogo, moneo, impero, curo); impersonals such as fit, accidit, futurum est, restat, sequitur "it happens, remains, follows that"; and oportet, necesse est.
It also spells out a noun or a pronoun that points ahead: Cura haec … ut … peragas "this concern, that you carry out …"; in hoc consistit, ut … "consists in this, that …"; finis … hic est, ut … "the end is this, that …".
Translate with "that" or "to": incumbat tibi, ut … peragas "let it press on you to carry out". The negative is ne after verbs of willing and caring, but ut non after fit and the like.
After oportet, necesse est, volo and licet the ut is often left out: advertas oportet "you must notice", agant necesse est "they must fare".