I would like to create a standard square groove in the plane, just like this.
But when i switch to object mode, it just almost disappear because I used subdivision.

I would like to create a standard square groove in the plane, just like this.
But when i switch to object mode, it just almost disappear because I used subdivision.

The solution is:
1. Select the edges you don't want to be subdivided

In the right bar (Press N to show it) set the "Mean Crease" value to 1 (it sets how much the selected edges don't react to subdivision smoothness Edit: note that the face is still subdivided but the subfaces created by subdivision mantain the original position. No smoothness.). They'll become violet.

To solve this:
Make Edge/Face as shown below: