Sotah chapter 9, mishnah 10. This mishnah lists the enactments and institutions established by Yochanan Kohen Gadol, and we will examine each one together with the reason behind it.
"He'evir hoda'as ma'aser":
His first enactment was the abolition of the declaration of the tithe. The declaration of the tithe is a text that a person would come up and recite in the Beis HaMikdash once every three years - at the end of the third year and at the end of the sixth year of the shemittah cycle - in which he details how he gave the various tithes. The reason for the abolition: one part of the text could no longer be said - "vegam nesativ laLevi" - and I have also given it to the Levite. From the days of Ezra, in response to the fact that the Levites (or at least most of them) remained in Bavel and did not go up with him to Eretz Yisrael, Ezra penalized them by ruling that they would not receive the first tithe, and the first tithe was given to the kohanim in their place. Since it was no longer possible to say "vegam nesativ laLevi," Yochanan Kohen Gadol abolished the practice of reciting the declaration of the tithe.
"Af hu bitel es hame'orerin":
Who are the me'orerin? These are the Levites who would sing in the Beis HaMikdash at the time the offerings were brought, reciting the verse in Tehillim: "Urah lamah sishan Hashem" - Awake, why do You sleep. The reason for the abolition: another verse says, "Hineh lo yanum velo yishan shomer Yisrael" - the Holy One neither sleeps nor slumbers. That first verse was said specifically at a time when the Jewish people are in distress and under the hand of the nations; but in the days of the Second Temple, even though they were not fully independent, their situation was to a certain degree a positive one, and at a time when the Beis HaMikdash is standing such a recitation is not fitting. He therefore stopped the Levites from saying "Urah lamah sishan Hashem."
"Ve'es hanokfim":
The nokfim are those who strike and beat. This refers to striking the animals brought as offerings, mainly cattle and the larger animals: they would strike them so that blood would flow into their eyes, so that they would be disoriented and easier to slaughter. Said Yochanan Kohen Gadol: this makes it look as though the animals are blemished. Even if technically it is not a blemish, that is how it appears, and this is not proper. He therefore installed rings in the ground, so that the animal's neck could be placed in them and it could be held that way.
"Ad yamav hayah patish makeh biYerushalayim":
This refers to the blacksmith's hammer, and it is speaking about chol hamoed. On chol hamoed it is permitted to perform work that is a case of potential loss, work whose postponement would cause monetary loss, and included in this was the permission for blacksmiths to strike with their hammers. But since this is an extremely noisy activity, and it is not fitting for it to take place in Yerushalayim on chol hamoed when the entire Jewish people are there, Yochanan Kohen Gadol abolished it - even for work involving potential loss, even for things that by strict law were permitted.
"Uvyamav ein adam tzarich lish'ol al hademai":
One who buys produce from an am ha'aretz does not know for certain whether tithes were separated from it or not. True, most amei ha'aretz do separate tithes, and nevertheless it was the practice to investigate and clarify the matter. Yochanan Kohen Gadol came and ruled that no investigation is necessary, but rather one uniform, fixed practice:
First tithe: he separates it and keeps it for himself, since he can say "hamotzi mechavero alav hara'ayah" - the burden of proof is on the one who makes the claim - and if a Levite, or perhaps a kohen, comes to demand the first tithe, he says to him: prove that the first tithe was not separated from this produce.
Terumah of the tithe: he separates it from the first tithe and gives it to a kohen.
Second tithe: he separates it and brings it up to Yerushalayim, or redeems it.
Tithe for the poor: even in the years of the tithe for the poor he need not separate it, and he is believed when he says that the am ha'aretz already took the tithe for the poor.
This leniency is possible because the two things he is required to separate are light burdens: terumah of the tithe is a small percentage, one hundredth of the total, while the second tithe goes toward the eating of the one who separates it himself, even though he must eat it in Yerushalayim. For this reason he instituted this enactment, and he no longer required a full clarification as to whether that am ha'aretz had taken tithes or not.
In summary: in this mishnah we listed the enactments of Yochanan Kohen Gadol: the abolition of the declaration of the tithe, because it was no longer possible to say "vegam nesativ laLevi" after Ezra's penalty; the abolition of the me'orerin, so as not to say "Urah lamah sishan Hashem" at a time when the Beis HaMikdash is standing; the abolition of the nokfim and the installation of rings in the ground, so that the animals would not appear blemished; the abolition of the hammer striking in Yerushalayim on chol hamoed, even for work involving potential loss; and the establishment of a fixed practice regarding demai, which exempts one from investigation and clarification.