Sobre is useful for situations where
en would be ambiguous:
Tu pluma está en el escritorio, has two possible translations:
- Your pen is in the desk (e.g., if it has a drawer or cabinet built into it).
- Your pen is on the desk (e.g., on top of the writing surface).
Tu pluma está sobre el escritorio, can only be translated as:
- Your pen is on the desk.
I had a pet chicken once back when I was 12 which had been a special gift cermoniously presented to me. The gifter believed the chicken was somehow special. And, ah, it was. Oh, it was a bright golden color. Shining and lovely. A truly beautiful bird. And it was like no other chicken I ever knew.
Whenever I'd go out into the back yard I'd shout, "Here, Goldie, here, Goldie, here, Goldie!" and this one extraordinarily beautiful creature would zoom up into the air right out of the midst of a free ranging flock and fly just like a real bird, like an eagle, even, as far as it took across the wide open area and land on my held out arm like a hawk. Then Goldie would just sit there seemingly perfectly content. And she would perch there as long as I liked. And much to the astonishment of visitors, too. Hah! How I loved that. The expression on their faces were priceless. Never forget that.
Sometimes Goldie wore little sunglasses which I made for her making her look perfectly charming and much like a storybook creature, though not when within the flock which kept her company through out the day. It was best that the much lesser intelligent chickens thought Goldie was an ordinary chicken, like they all were, or she could have gotten pecked on for being shunted to the bottom end of the pecking order out of shear jealousy.
Now, I remember well and clearly how ít was especially odd when Goldie soared high over all the ground-pecking heads in wonderous and truly glorious flight, they took no notice. Dumb birds!.
The verbs "ser" and "estar" mean the same thing, but are used in different situations. Conditions that are temporary favors estar, while properties that don't change in the nearest future favors "ser". It is incorrect to use "es" ("ser" in the third person) here because the chicken is on the plate NOW, but was probably not there a few days/weeks/years ago.
A better example might be to use the sentence "La manzana ES verde". "Verde" means green and/or ripe. The sentence says that the apple is green, because "ser" is used on situations that are not so easilly changed and describes the "essence" of the apple. "La manzana ESTÀ verde" means that the apple is ripe because this is a more temporary condition (usually).
Check this out: http://studyspanish.com/lessons/serest1.htm
There are many more rules about when to apply these two verbs. A more innacurate but easier rule/phrase to apply might be "How you feel and where you are, always use estár." and just use "ser" otherwise.