Negaim, Chapter 11, Mishnah 6. The whole of this perek treats nigei begadim, the tzoraas that breaks out on clothing. Before we begin the mishnah itself, it pays to open a Chumash to the closing verses of Parshas Tazria, where these halochos are set down, because here the Torah Shebal Peh rests entirely on the Torah Shebichsav, and every clause of our mishnah is simply an unfolding of those pesukim.
To place the mishnah, we need the picture that precedes it. A kohen came and saw on the garment one of the disqualifying colors: "adomdom," a strong red, or "yerakrak," a deep green. He declared hesger, and the garment was put away for a week. At the end of that first week the kohen returns and finds the color exactly as it was, still that deep red or deep green. This is the case of "omeid barishon," the nega standing unchanged after the first week.
Omeid barishon and omeid basheini
In that case the mishnah rules "yechabes," the garment is washed. Two points are worth noting. It is not clear that the kohen himself must be the one to do the washing, and it is not the entire garment that is washed but the area of the stain itself. After the washing comes "veyasgir," the kohen closes the garment away for a second week.
"Omeid basheini": should the second week close with the garment looking exactly as it did before, the disqualifying shade still sitting there, then "yisoreif," it goes to the fire. Note that with begadim the Torah never speaks of hechlet, a formal pronouncement of confirmed tumah with everything that follows from it. The garment is burned, and there the matter ends.
What corresponds to hechlet in the case of garments is spreading. If there is pisoyon, the stain expanding past its original borders, it makes no difference whether the kohen discovers this when he returns after the first seven days or after the second: "hapoiseh bezeh uvezeh yisoreif." In either situation the garment is burned.
Kohoh batchilah: the color faded before hesger
Now the mishnah turns to the opposite development, where the color weakens rather than holds or spreads. "Kohoh batchilah": the kohen arrives, and before he has been able to issue his psak of hesger, he sees that the color has already faded. Bear in mind that hesger is itself a psak of tumah; here the fading happened before the kohen ruled that the garment was tomei at all.
Rebbi Yishmoel says "yechabes veyasgir." We treat the garment as if it had reached the end of a first week, since the kohen did in fact see the original color and then saw a change in it. So the stain is washed and the garment goes into hesger.
"Vachachomim omrim ein zokek lo." The Chachomim hold that if no psak of hesger, no ruling of tumah, was ever issued on this garment before the deep adomdom or yerakrak faded to a lighter shade, then "ein zokek lo." The Rav explains: "ein zokek lo hasgiro vetohor." There is no obligation to place it in hesger and treat it as tomei to be watched; the garment is completely tohor.
Kohoh barishon and kohoh basheini
"Kohoh barishon": when the kohen comes back after seven days and finds that the strong red has dulled into a paler red, or the strong green into a paler green, the ruling is "yechabes veyasgir." The spot is laundered, and the garment must be shut away for another seven days.
"Kohoh basheini": this clause covers two possible histories. Either the shade held firm when the first week ran out and then weakened by the time the second was over, or it had already weakened after the first seven days and stayed that way through the second. Remember throughout this discussion that weakening means the shade is red still, or green still, merely paler than it was. Here the halochah is that he tears the affected spot out of the garment, "vesoreif mah shekohoh," burning the piece he removed.
Vetzorich matlis: why a patch is needed
Once he has excised that piece, which began as a strong adomdom or yerakrak and then weakened over the course of the two weeks, and consigned it to the fire, the mishnah continues: "vetzorich matlis," a patch must be sewn into the gap he made. The source is the word "oad" in the pasuk, "ve'im teiroeh oad babeged vesorafas." As the Rav puts it, "ve'ein oad ela bimkomo": "oad," once more, teaches that this identical spot has to remain fit to become tomei a second time, and a spot cannot be fit for anything unless there is fabric there. Hence the patch.
Rebbi Nechemya takes the other view: "eino tzorich matlis," no patch is called for at all. The halochah follows the Chachomim, who insist that one be inserted, and their support is the phrase "ve'im teiroeh oad," that the nega may yet appear a further time.
That leaves an obvious question, which the Tosfos Yom Tov raises: if Rebbi Nechemya does not learn the patch from "oad," what does he do with the word? The answer is that he needs it for the derashah of the braisa in Toras Kohanim. The braisa asks on the word "babeged": "yochol kol shehu," perhaps when the nega reappears on the garment a second time, in another spot, even the tiniest amount is enough. "Talmud lomar oad": just as the first appearance required the size of a gris, so too the second appearance requires a gris. The Rabbonon, for their part, learn two things from "oad" (tarti shema minah): that a gris is always required, and that the place cut away must be replaced with a patch. This is quoted in the name of the Maharam m'Rottenburg.
So the mishnah takes us through every way a garment placed in hesger can turn out: a shade that holds firm, a shade that spreads, and a shade that weakens, whether that weakening shows itself before the kohen has issued any ruling at all, once the first seven days are up, or once the second week has run its course. The closing scene is the cutting away of the affected piece of cloth, its burning, and the patch sewn in so that the spot stays ready to be seen once more.