Inside --- is more easily understood as being inside a house building box etc within --- is more easily understood as being inside a boundary or contained 'the man is within the confines of the university building" = the man is inside the university but it could be anywhere in the gardens or in any room. Somewhere inside the boundaries of the university. Just in = close to the boundary or just over the threshold (doorstep) of a house or building Just in -- is often used in sports