Shabbos, Chapter 13, Mishnah 3. As we mentioned earlier, all the labors of Shabbos - all the forbidden activities - must be done in a way that builds and improves, and not in a way that destroys and ruins. On this basis the Mishnah rules:
"HaKorei'a bachamaso" - one who tears, which is one of the labors forbidden on Shabbos, but does so out of anger.
"U'va'avlo" - one who tears out of emotional distress over the loss of a loved one.
"V'chol hamekalkelin peturin" - and likewise any other destructive act, which involves no building or improving.
And as we mentioned in the previous Mishnah, one who tears is liable only when he does so for a constructive purpose, the example being one who tears in order to sew two stitches. Sometimes it is necessary to tear something so that it can be remade properly, and such tearing is considered constructive.
The Gemara's qualification regarding tearing over the deceased:
The Gemara clarifies that when the Mishnah speaks of one who tears his garment over someone who has passed away, over one who was close to him, and considers this a destructive act that ruins - this refers only to a relative for whom there is no obligation to tear. For the Torah obligates a person to tear his garment over someone for whom he sits shivah, and to be considered destructive the act cannot be the performance of that very mitzvah. Therefore, one who through his tearing fulfills the mitzvah that he is obligated in is considered to be improving, and he would be liable for it on Shabbos. Of course, one who has lost a family member does not tear on Shabbos, but only after Shabbos.
"V'hamekalkel al menas letaken":
The Mishnah continues and rules: "V'hamekalkel al menas letaken - shiuro kimtaken" - one who ruins in order to repair, his measure is like that of one who repairs. Whenever a person performs a destructive act for the sake of something constructive, as was mentioned in the previous Mishnah - one who tears a piece of cloth in order to sew it anew - the measure that makes him liable is gauged according to the constructive act he intends to perform afterward. In the case of tearing, he must tear enough to allow for sewing two stitches, and that is the measure.
The same rule holds true for other labors as well. For example, one who erases two letters, who is liable on Shabbos, must leave in the erased space enough room to write two letters, since that is the constructive purpose for which the erasing is done.
In summary: In this Mishnah we learned that one who tears out of anger or out of mourning, and likewise all who ruin, are exempt, since liability for a labor applies specifically to a constructive act. We noted the Gemara's qualification, that one who tears over a relative for whom he is obligated to tear is considered to be improving, because through this he fulfills a mitzvah. And finally we learned the rule "v'hamekalkel al menas letaken - shiuro kimtaken": the measure that creates liability is determined by the constructive act for whose sake the ruining was done - tearing enough to sew two stitches, and erasing enough to write two letters.