back means back home/ back to a place, a person etc. (= go back, come back, get back, take something back etc.) (e.g. What time will you be back?)
back somebody means support (e.g. Tom backed me up over my decision to quit my job.)
back something up means reverse (e.g. I’ll have to back up my car so that you can get out.)
be off (to a place) (e.g. Tomorrow I’m off to Paris. I’m off on holiday)
beat somebody up means hit someone repeatedly so that they are badly hurt (e.g. A friend of mine was attacked and beaten up a few days ago.)
blow out a candle (e.g. We don’t need the candle. You can blow it out.)
blow something up means add air (e.g. We have to blow hundred balloons up for the party.)
blow up means explode, blow something up means destroy it with a bomb etc. (e.g. The racing car blew up after it crashed into the fence.)