Fixing an item of clothing sometimes requires that it be torn in order to be re-sewn. This tearing constitutes the melakha of Kore’a. Tearing in order to sew is a violation of a Torah prohibition. In the Mishkan, worms would sometimes chew holes in the curtains. Mending the hole as is would leave the curtain rumpled. Therefore, the curtain needed to be torn apart and re-sewn.
Purposeful, constructive (“le-to’elet”) tearing is prohibited by Torah law. One example of this is taking down a hem in order to lengthen an item of clothing. Non-purposeful, destructive tearing is prohibited rabbinically.
Tearing plastic bags or plastic tablecloths from a roll in order to use them is prohibited by Torah law. Similarly, tearing toilet paper in order to use it is prohibited by Torah law. Some maintain that tearing along the perforations is also a transgression of the melakha of Meĥatekh. Tearing toilet paper with a shinui is a rabbinic transgression. At a time of need, such as in order to avoid serious embarrassment, the Sages permit violating their ordinances. Therefore, one who finds himself in a situation where he cannot wipe himself without tearing toilet paper may tear it in order to preserve his dignity. He should do so with a shinui, such as using his elbows to pull the paper from the roll. He should be careful not to tear along the perforations (SA 312:1; MB ad loc. 12; SSK 23:19; Orĥot Shabbat 11:40).
If a book has some uncut pages, it is forbidden by Torah law to cut them on Shabbat. If the pages were cut properly, but they became stuck together by a stray bit of glue, they may be separated; this is because they were stuck together accidentally, with no intent of permanent attachment (MA 340:18; MB ad loc. 45). One may not pull apart tissues that were not cut properly and thus are still partially joined together at some points.
Just as attaching papers or pieces of cloth is a tolada of Tofer, separating these attached items is a tolada of Kore’a. Therefore, one may not separate pages that were stapled together. Similarly, one may not tear out a page from a writing pad.
One may tear open a bag containing food, just as one may peel an orange in order to eat it. This is because the tearing or peeling is not for the sake of the bag or the peel, but in order to eat what is inside. Similarly, one may open the top of a bag of sugar that is glued shut. Some are stringent and do not allow this, but the lenient position is the primary one (below 15:12).
If one did not manage to remove the manufacturer’s tags from a new item of clothing before Shabbat, he may cut the plastic thread connecting it to the garment on Shabbat; the tags are only loosely attached to the clothing and thus are not considered sewn to it.
. According to BHL 314:8 s.v. “ĥotalot,” there is no prohibition here of tearing a thread, since it is only tearing in order to sew that is prohibited by Torah law. While the Sages prohibited even tearing that is not for the purpose of sewing, they did not prohibit tearing a thread that is impossible to sew. Nevertheless, one may not tear a thread that is attached to the edge of a garment, because doing so is considered applying the finishing touch on the clothing, a violation of Makeh Be-fatish. This follows the stringency found in Menuĥat Ahava 3:16:8, in contrast to the leniency found in Orĥot Shabbat ch. 11 n. 26.If a dry cleaner stapled a tag to a garment in such a way that the tag is not visible, then it should not be removed. If it is visible, and people normally would not go out with it visible, at a time of necessity one may rely on those who maintain that this is an impermanent type of sewing, and remove it. (See Rema 317:3; MB ad loc. 21; Yeĥaveh Da’at 6:24.)
Opening a letter: The poskim disagree whether one may open a sealed letter on Shabbat when there is a chance that its contents are relevant to Shabbat. According to Pri Ĥadash, Ĥayei Adam, and MB 340:41, this is forbidden because of Kore’a. However, according to Maharil, Taz, and MA, this is permitted because its seal was meant to be temporary. In a time of need, one may open such a letter as long as he makes a point of ripping it open and thus destroying the envelope (Ĥazon Ish and Yeĥaveh Da’at 6:24).