Shabbos, Chapter 2, Mishnah 7. Our Mishnah shares with us the actions and instructions that need to be taken care of before Shabbos begins, in addition to lighting the candles. There are three things a person must say in his home on Erev Shabbos as darkness falls: two of them are questions, and the third is a statement.
The three things to say on Erev Shabbos:
"Isartem?" - "Have you tithed?" One may not separate tithes on Shabbos, so if a person wants his food to be fit to eat, he must make sure it is not tevel and that the tithe has already been separated from it. This is especially so on Shabbos, when even a casual bite, a mere taste, is forbidden without tithing, because Shabbos establishes food as a fixed meal for the purposes of tithing: once Shabbos arrives it renders the food a fixed meal and obligates separating the tithe even for a casual bite. For this reason it is very important to tithe in advance, before Shabbos.
"Eiravtem?" - "Have you made the eruv?" This refers to eruv techumin, for someone who needs to walk on Shabbos to a place beyond the techum, and also to eruvei chatzeiros, for someone living in a courtyard who wishes to carry within it, who must make an eruv together with the other residents of the courtyard. One must therefore verify that these things were done before Shabbos.
"Hadliku es haner" - "Light the candle." The third item is not a question but a statement to the members of the household: the time has come to light the candles. It is not phrased as a question because the candles are visible to the eye, whereas the eruv and the tithe are not noticeable as to whether they were done or not.
Bein hashemashos:
The Mishnah goes on to teach the law regarding that period of time on Erev Shabbos when it is unclear whether Shabbos has already begun or not, the time known as bein hashemashos (twilight). In the words of the Mishnah: "safek cheshechah" - it is uncertain whether full night has already arrived, and uncertain whether it is perhaps still not night.
During this period the following are forbidden:
Separating tithes from something that is definitely tevel - a rabbinic prohibition, because it appears like repairing an object.
Immersing utensils in a mikvah - this too resembles the matter of repairing, and it too is rabbinic.
Lighting candles - a Torah prohibition, and certainly forbidden at bein hashemashos.
On the other hand, the Sages were lenient during bein hashemashos, which is a time of uncertainty, in three matters:
"Me'asrin es hademai" - "One may tithe demai": produce purchased from an am ha'aretz, who is not presumed to be careful about the laws of tithes. Because of this concern one must be stringent and separate tithes again, and this separation is permitted at bein hashemashos.
"Ume'arvin" - "And one may make an eruv." This seems difficult, since at the beginning of the Mishnah it says that one must ask beforehand whether they made the eruv, implying that one may not make an eruv at bein hashemashos. The Gemara explains that the reference here is specifically to eruvei chatzeiros, which are only rabbinic and therefore treated more leniently, so making them is permitted. But eruv techumin - which according to several opinions is from the Torah, and according to others is at the very least a rabbinic law with support in the verses - may not be made at bein hashemashos.
"Vetomnin es hachamin" - "And one may insulate hot food." It is permitted to insulate with a material that does not add heat.
And even though the Sages generally forbade a person to insulate his food on Shabbos, at bein hashemashos they nevertheless permitted it. The entire concern with insulating on Shabbos is that a person may find that the food is no longer hot and will come to stir the coals or the like, and this concern does not exist at bein hashemashos, because at that hour the assumption is that all the food is still hot. Only at a later hour might the concern arise that the food is no longer hot and that the person may be tempted to stir the coals - but not at bein hashemashos.