If one forgot to perform Eruv Tavshilin:
If one forgot to perform Eruv Tavshilin on Erev Yom Tov and it is already after nightfall [Tzeis Hakochavim] then by a regular Yom Tov that falls on Thursday-Friday one may still do so through making a Tnaiy. However on Rosh Hashanah one may no longer do so even through making a Tnaiy, as explained above in B. Likewise when any Yom Tov falls on Friday-Shabbos, a Tnaiy does not help. One is to speak to a Rav in regards to how he may cook food on Erev Shabbos for the sake of Shabbos.