I need the connection between dense(see <https://ncatlab.org/nlab/show/dense+subcategory>) and reflective (<https://ncatlab.org/nlab/show/reflective+subcategory>) subcategories, i.e. If $C$ is a dense subcategory when $C$ also is reflective? 
or if $C$ is reflective then $C$ is dense?



Dense and reflexive subcategories
There are dense subcategories (in the sense that every object is a colimit of objects in them) which are not reflective. Take R-mod, the finitely presented right R-modules. This is dense in R-Mod (the whole category of modules) as every module is a directed colimit of finitely presented modules. It is not reflective in R-Mod, as R-Mod is complete: A small category (R-mod is essentially small) cannot be complete unless it is a pre-order (R-mod is not a pre-order unless R=0), yet if R-mod was reflective in R-Mod it would be complete (in fact, it would be closed under limits).


0 is reflective in R-Mod but certainly not dense!


