Tevul Yom, Chapter Four, Mishnah Four
1) A flagon was a tevul yom and they filled it from a cask containing tithes from which terumah had not yet been taken:
2) If one said, let this be terumah of tithe after nightfall, it becomes terumah of tithe.
3) But if he said: let this be the food for the eruv, he has said nothing.
4) If the cask was broken, the contents of the flagon still remain tithe from which terumah had not yet been taken.
a) If the flagon was broken, then what is in the cask still remains tithe from which terumah had not yet been taken.
Section one: There is a cask of tithe of some sort of produce. The terumah which must be taken from tithes has not yet been removed from the produce. They then filled up a flagon which was a tevul yom (it had been immersed but the sun had not yet set). The question ishow to separate terumah without defiling it by contact with the tevul yom flagon?
Section two: The solution is for the person to say that the terumah of tithe will only come into being after nightfall. In such a way the terumah will remain pure because by then the flagon will be pure.
Section three: In this section, the person wants to use the contents of the flagon as his eruv, the shared meal that will allow him to carry in a courtyard. The problem is that the eruv must be placed before the sun sets and before the sun sets this produce is still tithe from which terumah has not been removed, which cannot be used for an eruv (see Eruvin 3:2). Therefore, if he tries to use this as his eruv, it doesn’t work.
Section four: If either the cask or the flagon breaks before the sun sets, before the statement he made in section two has had time to become operative, then the contents of the other container remain tithe that has not had terumah removed. In other words, until the sun sets his statement doesn’t succeed in separating terumah from the tithe.