adjective
BAD : The car could not stop because its brakes were broken.
GOOD : The car could not stop because its brakes were not working.
BAD : I turned on the radio but it was broken.
GOOD : I turned on the radio but it wasn't working.
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broken = physically damaged: 'a broken windscreen'
not working = not functioning properly: 'I can't ring him because his phone's not working.'