(i'm not very advanced so please bear with me)
when learning beitzah with my rebbe, we learned that if an eruv tavshilin is made & the eruv is eaten on YOM TOV, rashoos in his hand is given to COOK,BAKE,INSULATE ON SHABBOS. i asked my rav if this is the halacha since in my readings/learnings over the year I seem to have thought if the eruv is eaten, spoils, goes lost etc one CAN NOT use it for shabbos? the rav looked it up & clearly without hesitation stated that most people DO eat on shabbos, especially shlash seudas but & that might be ideal but clearly one can bake, cook, insulate & light shabbos candles (even though bracha says you need the eruv for these) Then when i researched via some seemingly legitimate internet sites (aish hatorah, etc) it said the opposite. My rav said he doesn't like when they posken stricter without mentioning bidieved/lachatchilla etc for perhaps balaay tshuvah or not yet fully educated & affiliated since it makes it confusing. Afeter reviewing what the gemara , my rav & others say. I th!
ink it might mean that if one fully ate the eruv on YOM TOV completely & he already had at least begun his preparations, then it is ok, but if he ate the eruv on YOM TOV & after that he decides to prepare for shabbos but hadn't yet even beun ANY preparation (even a preliminary act , then it is prohibited) Is this correct? It seems since the eruv is rabbinic in the first place it should be more lenient should we eat it on YOM TOV?
yehoshua yosef fox, BKLYN, USA
The Shulchan Aruch (O.C. 527:15) states, "If the Eruv was eaten or lost before he cooked for Shabbos, he cannot cook unless a k'Zayis was left." The Mishnah Berurah (#47) notes that even if the entire bread used was eaten/lost it does not matter, as long as a k'Zayis of the Eruv was left.
Technically, the Eruv is not needed once someone has finished preparing for Shabbos (although some say it includes lighting candles for Shabbos as well, and therefore it is appropriate to keep it extant until at least sunset on Friday night). It is an appropriate custom to use the bread as Lechem Mishnah for all Shabbos meals and to finally eat it at Se'udah Shelishis (see Mishnah Berurah #48).
The Shulchan Aruch (O.C. 527:17) does mention that if the Eruv was completely lost/eaten while preparing a dough or cooked dish, one can finish preparing that item, even if it involves putting it on the fire. However, he clearly cannot start preparing other items based on this Eruv after it has been fully eaten/lost.
All the best,