This coming holiday season (Tishri 5774), we will be required to prepare an Erub Tabshilin three times – on Ereb Rosh Hashanah, Ereb Sukkot, and before Shemini Aseret.
The Shulhan Aruch writes that the Erub Tabshilin must consist of two food items – one baked food, and one boiled food. The common practice is to use bread for the baked item and a boiled egg for the boiled item. The Shulhan Aruch adds that if, for whatever reason, one did not set aside a baked item, and prepared only a boiled item for his Erub Tabshilin, his Erub is nevertheless valid, since the primary component of the Erub is the boiled food. And thus if a person just took a boiled egg and recited the Erub Tabshilin text, this suffices as his Erub. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) clarifies that in such a case one may cook and bake on Yom Tob in preparation for Shabbat. Although the bread component of the Erub serves to permit baking in preparation for Shabbat, Be’di’abad (after the fact) one may cook and bake as long as he prepared a boiled food for his Erub.
The Mishna Berura further notes that if one remembers before the onset of Yom Tob that he forgot to prepare a baked item, then he should take a baked item and add it to the Erub, and then repeat the "Haden" declaration. He does not, however, repeat the Beracha.
In the reverse case, where one prepared only a baked item and not a boiled item, the Erub is not valid. According to most Halachic authorities, in this case one may not even bake on Yom Tob in preparation for Shabbat, since he did not prepare the primary component of the Erub.
If one knows that his wife will not be baking on Yom Tob in preparation for Shabbat, then he does not have to prepare a baked food for the Erub. Since, as mentioned, the baked food serves to permit baking, one who knows that baking will not be necessary may Le’chatehila (from the outset) use only an egg for his Erub Tabshilin. This is quite common, as most women do not start baking cookies and cake – which is very time consuming and makes the kitchen disorderly – on Yom Tob. Usually, women prepare all the baked goods ahead of time and put them in the freezer. If this is the case, it suffices to use a boiled egg for the Erub Tabshilin, without a baked food item.
Summary: The Erub Tasbhilin must consist of a baked item – usually a piece of bread – and a boiled item – usually a boiled egg. One who knows that his wife will not need to bake on Yom Tob in preparation for Shabbat may use only an egg. If one does need to bake and mistakenly used only an egg, his Erub is nevertheless valid and he may bake and cook on Yom Tob for Shabbat. But if he prepared only a baked item, he may not cook or bake on Yom Tob in preparation for Shabbat.
