Shel Yad: Protector Stuck
Question
I put my tefillin shel yad protector on wrong and forced it on and now it is completely stuck and I don't know if i damaged the corners
Answer
Shalom!
Thank you for your question.
When you finally remove the protector, examine the corners of the Tefillin.
It is a “halacha l’moshe m’sinai that tefillin are to be square in their length and width. This is true for both the shel yad and the shel rosh. If the squareness was ruined the Tefillin must be fixed.
As long as the corners of the Tefillin look square to the naked eye, the Tefillin are kosher, although nowadays people make the Tefillin with very sharp corners. Regarding the edges along the width of the Tefillin, if the dent is deep enough that a fingernail would get caught in it, it should be fixed. One may only fix the ribua with the actual bayit, not by applying paint (tsevah).
If in doubt one should show his Tefillin to a rabbi or Sofer.
Source
Mencahot 35a; OC 32 Halacha Pesukah Page 122