Ago = in the past. I was living in China a year ago. I went to Hong Kong about a year and a half ago. How long ago did you start to study English? I was young long ago, now I'm old. Before = sooner than something else; earlier in time than some other event or happening. I want to see you again before I leave. Before I visit China again I need to get a visa. I lived in America before I lived in China. I was young before I got old. Good luck to you!

Ago refers to a long duration of time whereas Before refers to short duration of time The second world war began BEFORE 1940 The second world war started over 60 years AGO