Translation:Shulchan Aruch/Orach Chaim/261

1 When there is a doubt as to whether it is dark, and this is bein hashemashos (this is the amount of time to walk 3 fourths of a mil after sundown (Tur in Siman 293; and as below, end of Seif 2) [and the measurement of a mil is a third of an hour minus a 30th] we do not tithe what is certain, and we do not immerse the vessels, and we do not light the candles, and we do not establish the eruv techumin (and see below, Siman 415, Seif 2). However, we do tithe what is uncertain, and we insulate the hot food, and we establish the eruv for the courtyards. (And see below, Siman 393). And it is permissible to say to a heathen during bein hashemashos to light a candle for the need of Shabbat. And likewise, to tell him to do any work that is for the need of a mitzvah or that it is a taxing and pressing matter. Gloss: And likewise, one who accepted Shabbat upon himself an hour or two before dark can tell to light the candle and to do other things that he needs [Mahari"v, No. 116; and see below, Siman 342].

2 There are those who say that one must add from the mundane to the holy. And the time of this addition is from the beginning of sunset, when the sun is not visible on the earth, until the time of bein hashmashos. And this period, that is equivalent to the time of three-and-a-quarter mil, if one desired to deem its entirety as an addition, he does so. If he wanted to add part of it, he does so. And this is provided that one will add some amount of time when it is certainly day from the mundane to the holy. And the time of bein hashemashos is 3 fourths of a mil, which is a walking distance of a thousand and 500 cubits before nightfall.
​Gloss: And if one wishes to do this earlier and to accept Shabbos upon himself from Plag Hamincha and onward, he may do so. (Tur; and Agur in the name of Tosefos, Chapter "The prayer of Shaharith") [and see below, Siman 267].

3 And one who is not knowledgeable regarding this measure should kindle the lights while the sun is still visible on the treetops. And if it is a cloudy day, he should kindle the lights when the roosters sit on the beam while it is still day. And if he is in a field where there are no roosters, he should kindle the lights when the ravens sit while it is still day.

4 After the answering of 'Barchu', even though it is still day, one does not make an eiruv and does not insulate because they accepted Shabbat on themselves. And for us, the recital of 'Mizmor Shir l'Yom Hashabbat' it is like the answering of Barchu for them.