So, I am making a shelf kind of thing and i have started modelling using a simple cube, but i want to have a curve only on one side of the cube, I tried with fillet but it does that on all the four sides of the cube. Is there any other way to do it ? I also tried Sub-divisions, but it makes it too smooth. I just want a simple curve, are there any other ways the give the fillet on one side of the cube?
You are looking for Bevels. If your cube is editable you can use the Bevel Tool (in Edge mode) and if the edge you want is selected it will bevel only that edge. You can also use the Bevel Deformer which will do the same stuff and can be restricted by a selection tag to operate on only one edge (or any number of edges). The Bevel deformer can also work on a parametric cube and can even work on a single edge of a parametric cube by first using a Correction deformer to select an edge, saving that as a selection, then using that selection in the Bevel deformer.
and for completeness - subdivisions can be controlled and can even have sharp edges (Via the SDS Weight tag) but control like you would want would likely also involve using the knife tool to add edges in the right places. In other words it's possible but the bevel tool or bevel deformer would be easier.