The blue Violets that flower in April are very much alike. See here the Common Dog Violet (Viola riviniana).
It look rather rectangular. It has to be admitted, violets do show a temperament.
This one grows in woods on neutre or slightly acidic soils in little rather loose bunches. If there is a leaf rosette of a kind, it is not at ground level but halfway up a stalk.
Common Dog Violet has a big white spur and pointed sepals...
... and a striped lower lip.
Here it grows between leaves of a Sweet Chestnut that, like the Violet, does not like calcareous soils.