פירוש על עבודה זרה 77:21

R. Yohanan says that even if we know that wine was put in the pickled foods, it is permitted to derive benefit from them. But R. Meir prohibited one from deriving benefit from murias. So how would he distinguish between the two?
The murias mixture has the presence of the wine—therefore it is of a higher degree of prohibition. But the wine in the pickled food is there in taste but not substance. Therefore it is only prohibited to eat this food; it is not prohibited to derive benefit from it.

The mishnah listed certain fish that may not be eaten. The Talmud explains what the hilak fish is. Since the size of the hilak is the same as other small, non-kosher fish, the hilak may not be eaten, lest other non-kosher fish are eaten with it.

As long as the fish will have fins and scales or once had fins and scales it is kosher. It does not have to have them when it is eaten.

R. Abahu assumes that fish-entrails and eggs bought in Caesarea are kosher because they come from places where only kosher fish are caught. This is similar to Abaye who assumes that the small tzahanta fish from a certain river are also kosher.

The Talmud tries to understand why Abaye is so sure that unclean small fish do not live in the river Bab-Nahara. The answer is that the conditions simply are not conducive for their growth. The rapid water and its saltiness are not a problem, but the mud is. Ravina revokes Abaye’s leniency—since two other rivers now join the Bab-Nahara, we cannot assume that the fish is kosher.

Here rabbis list fish and provide some mnemonics to remember which one is kosher and which one is not.

In all of these cases the rabbis found a way of determining that the fish do indeed have scales and are thus permitted.

Rabbah b. Bar Hannah gets lucky and hears someone call the permitted “zahanta” fish “batei.” Since “batei” is the name of a non-kosher fish, the rabbi did not eat it.

Hiltit has a sharp taste, and so when it is cut with a knife which has been used to cut nonkosher substances, the taste of the non-kosher substances will go into the hiltit. And although such residue taste is generally considered as “imparting a worsening flavor” and permitted, in this case since the hiltit is sharp the flavor will turn out good. The implications from here are that when one cuts most foods with a non-kosher knife, the food is permitted. But not if the food has a sharp taste.

A “haver” is a term used in rabbinic literature for one who is scrupulous about observing various halakhot, mostly tithes and purity, but here it seems to be general laws of kashrut. Since R. Levi was a haver, people can trust his slave (or servant) to not cut hiltit with unkosher knives.

The blue wool is used to make tzitzit. The problem is that a specific type of dye must be used and it is easy to fake this dye. Someone who buys dyed wool needs to trust the seller that the dye used is the correct one. Ultimately, R. Huna learns that he can trust the wife of a haver.

This baraita supports what was said above. The presumption that a haver behaves scrupulously extends to his wife and slaves (or servants). It also continues after his death until people begin to have specific suspicions about him.

An am ha’aretz is a person who is by definition not a haver, meaning the person is not assumed to act with a high level of scrupulousness in certain matters such as tithing and purity.
To be trusted as a haver, meaning to move from being an am ha’aretz to being a haver, one has to formally accept upon himself or herself to act in accordance with these ways. Indeed, the laws of conversion were probably patterned after these laws. So if a woman who grew up as an am ha’aretz married a haver (pretty remarkable in and of itself) she must accept upon herself these responsibilities. So too with a slave joining a haver’s household. The trust her husband has does not accrue to her immediately.
But if a woman marries out—meaning she marries an am ha’aretz, R. Meir says that she maintains her reliability as haver. R. Judah says she does not. Since she grew up as a haver, she never had to accept this responsibility on herself. But now she does. R. Shimon b. Elazar cleverly illustrates how a woman can move from being married to a haver, considered an honorable, trustworthy person, can lose her level of piety by being married to a tax collector, a position considered somewhat disgraceful in rabbinic society, associated with robbery and violence.

The fear with milk, meat and cheese seems to be that kosher ones were switched with non-kosher ones. The fear with blue wool is that the non-Jew switched the wool with wool dyed with dye not kosher for tzitzit.
The hiltit may have been cut with an unkosher knife. The murias (fish hash) may have been switched with murias that has non-kosher fish in it. The wine may have been touched by the non-Jew, which would render it undrinkable. The issue with the bread is that it may have been switched with bread made by non-Jews, which is forbidden.
Rav requires two seals for some of these things, but only one seal for the others