Any melacha performed on Friday during bain hashmashos is forbidden to benefit from, just as if it was done on Shabbos. This applies even if the melacha was done by mistake. (MB6)
No shvus (issur d’rabbonon of Shabbos) was decreed during bain hashmashos if you need to do it for a mitzva. Therefore you may tell a non-Jew to light a candle for the sake of Shabbos. Even for a non-Shabbos need or non- mitzva need, you can tell a non-Jew to perform a melacha during bain hashmashos if it is a great need. (e.g. to light a yahrzeit candle so you won't be upset). The same hetter applies to a significant financial loss. (ShA1 – MB13 – BaH3 – BeH “l’hadlik”)
Once your community accepts Shabbos (early), you aren’t allowed to do even a shvus for a mitzva, you can only instruct a non-Jew to perform the melacha. However, as long as the tzibbur has not yet accepted Shabbos, you may even instruct another Jew to do any melacha. (MB17,18)
Another hetter during bain hashmashos is to make an eruv chatzayros or an eruv tavshillin. You can also do hatmanna in material that does not add heat during bain hashmashos. Again, this hetter applies only if you aren’t in a community, since communities accept Shabbos before bain hashmashos. (ShA 1 – MB10,24 – BeH “um’arvin”)
There is a mitzva to bring in Shabbos early (before bain hashmashos) and to end it late. It is called “tosefes” Shabbos since you “add” from the weekday onto Shabbos. (ShA2 – MB19)
When is bain hashmashos? There are 3 opinions:
The Shulchan Aruch is the famous Rabaynu Tam’s shita – after sunset, it is still day for another 3 ¼ mil (58 ½ minutes) – then bain hashmashos begins for the duration of another ¾ mil (13 ½ minutes) – 4 mil (72 minutes) after sunset is tsais hakochavim.
The Gr”a and the geonim (Beur Halacha quotes 16 poskim) – bain hashmashos begins at sunset and goes for ¾ mil until tsais hakochavim.
Rabi Eliezer Mimitz holds that bain hashmashos begins ¾ mil before sunset and ends at sunset. (ShA2 – BeH “mitchillas”)
What should be our conduct on Friday? Beware from doing melacha after sunset (because of the geonim quoted above) – do not even light Shabbos candles anymore. You must finish the lighting before sunset. And do not wait until the last minute, light candles when the sun is still shining on the tree tops. Fortunate is the one who is machmir to refrain from melacha a half hour, or at least 20 minutes before sunset to fulfill even the shita of Rabi Eliezer Mimitz plus a little bit of tosefes Shabbos. (MB23 – ShaH21)
How much time is tosefes Shabbos? The only thing that is clear from the gemara is that it is less than ¾ mil. Many achronim hold 2 minutes. The Mishna Berura quotes the Chayay Odom that tosefes Shabbos with bain hashmashos add up to about half hour. (MB22 – BeH “ayzeh” – ShaH21)
When is the earliest time for tosefes Shabbos? From 1 ¼ halachic hours before sunset. Before that is nothing. (Rama2 – MB25)
How do you accept tosefes Shabbos? Either by verbally saying so, or by saying “boi Kalla” in Lecha Dodi when recited before bain hashmashos. Women accept it by lighting Shabbos candles. (MB21,31)
On Motzo'ay Shabbos it is proper l’chatchila, to refrain from melacha until “Rabainu Tam” – 72 minutes after sunset. Some poskim say that during the summer, 4 mil is longer than 72 minutes. However, other signs are given to know that it is night – so if there is no more redness on the western horizon and there are 3 stars out, you do not need to wait past 72 minutes even according to Rabaynu Tam. (BeH “shehu”)
Those who rely upon the geonim still should not do melacha right after ¾ mil from shkia – wait until 3 small stars emerge in the sky. This siman can apply in any season and at any location. (BeH “shehu”)