if something does not happen► if not · Your car should be ready by 12 o'clock, but if not I'll let you know.· If you don't leave now, I'll call the police.· Try these gloves on. If they're not the right size I'll take them back.if not, why not? spoken (=used to ask why something has not happened or why someone has not done something) · Have you done your homework yet? If not, why not?
► unless use this to say that something will happen if something else does not change the situation: · Unless the weather improves, we will have to cancel the game.· You won't pass your examinations unless you study hard.· Milk quickly turns sour, unless it's refrigerated.
► otherwise use this when there will be a bad result if someone does not do something or if something does not happen: · Stir the sauce until it cools, otherwise it will be lumpy.· I'm glad you told me about the show being cancelled. Otherwise I'd have travelled all the way to Glasgow for nothing.
► or/or else use this when you are warning someone what will happen if they do not do what you are telling them to do: · Be careful or you'll bump your head.· Stop making so much noise or else the neighbours will start complaining.
► without use this when you cannot do something if you do not do something else first: · No one can succeed in business without taking certain risks.· How can you judge a book without reading it?
► barring use this when something will happen or continue in the way that you want, if something does not happen to prevent it: · Barring unexpected delays, work on the tunnel should be completed by the end of next month.
► before use this when you are saying what someone must do if they want to stop something bad from happening: · Put that money somewhere safe before it gets stolen.· That dog ought to be destroyed before it attacks any more children.
► failing that use this when you are saying what you will do if the first thing you suggested is not possible: · My mother wanted me to be a teacher or, failing that, a nurse.· Dr Schwabe said he could find me a room either on the campus, or failing that, in a house nearby.
unless something happens: 
Barring a miracle, he won’t walk again.