

Adapted by Rabbi Ariel Ovadia[1]
I forgot to separate Hallah before Shabbat from the bread that I baked. What should I do? Am I permitted to separate Hallah on Shabbat?
The Shulhan Aruch (OH 339:4) writes that one is not permitted to separate Terumot and Ma’asrot (various tithes) on Shabbat. Included in this prohibition is separating Hallah. The Mishna Berura (261:4) explains that Hachamim forbade separating tithes on Shabbat and Yom Tov since this “fixes” the foods and makes them useable. “Fixing” food is forbidden on Shabbat and Yom Tov, since it resembles the Melacha of Tikun Manna (the creative act of fixing a vessel). However, outside of Eretz Yisrael, one may eat bread before Hallah is separated, provided that Hallah will be separated later. Therefore, if on Shabbat one realizes that they did not separate Hallah, they should make sure to leave over a slice of bread, so they can separate Hallah after Shabbat.
I separated a piece of dough for Hallah, but it was accidentally mixed back with the rest of the dough. What should I do?
According to Maran (Bet Yosef, YD 323:1) there is a special leniency regarding Hallah of Hutz La’Aretz (outside of Eretz Yisrael), that as long as the piece of Hallah that was mixed back with the rest of the dough was smaller or of equivalent size to the rest of the dough, the entire mixture is permitted. Sefaradim may follow the ruling of Maran and eat this bread with no concern.
Ashkenazim follow the ruling of Rama who writes that if the piece of Hallah gets mixed back with the other dough, it is nullified only if the rest of the dough is one hundred times the volume of the Hallah. In a case where there is not enough dough to nullify the Hallah, one can annul his Neder (vow) and separate a new piece of Hallah.
I was given a loaf of bread by a neighbor and I am concerned that she did not separate Hallah. Is there anything I can do?
There is no need to ask the neighbor if she has already separated Hallah, especially if one understands that this might cause embarrassment. Hallah can be taken even after the loaf has been baked (Shulhan Aruch, YD 327:5). One should simply remove a piece from the loaf and, without reciting the Beracha, say “this shall be Hallah.” Although it is questionable if anyone aside from the owner of the loaf or their proxy can separate Hallah (see Yad Avraham, YD 328:3), in this case there is no need to ask permission. The loaf was given as a gift. You are now the owner and may take Hallah. In Hutz La’Aretz (outside Eretz Yisrael) one may even eat some of the loaf and separate Hallah afterwards (ibid. 323:1). However, one must leave over a piece which is large enough that there will still be something left over after separating the Hallah.
May Hallah today be eaten by a Kohen?
In Eretz Yisrael, where the obligation of Hallah mirrors the Torah obligation, it is forbidden in all situations for Kohanim to eat Hallah. This is because Hallah is Kodesh (holy) and may only be eaten by Kohanim who are Tahor (ritually clean). All Jews today are assumed to be Tamme (ritually unclean), and we lack the means to purify ourselves from Tum’at Met (impurity of the dead). Furthermore, in – almost – all situations, the Hallah itself is Tamme and must be burned.
Outside of Eretz Yisrael, the laws of Hallah are less stringent and, strictly speaking, a Kohen under the age of nine, or one who has immersed in a Mikve is permitted to eat Hallah. However, the Rama (OH 457:2) writes that some hold that it is best not to give Hallah to any Kohen, since there might be doubts as to the precise lineage of the Kohen. The Shach (YD 322:9) writes that the general practice year-round was not to give the Hallah to Kohanim, with the exception of Erev Pesah when Hallah was given to young Kohanim (see Magen Avraham 457:9 for explanation)[2].
Sources:
[1] Based on the OU Kosher bulletin
[2] The P’ri Megadim (457:9) writes that in his days, he never saw anyone give challah to a Kohen