Introduction to Eruvin
Tractate Eruvin is a continuation of tractate Shabbat. Most of Eruvin deals with the different types of domains: public, private and two in-between domains, which we shall explain when they appear. The categorization of property into different domains is relevant for the laws of carrying. The tractate also deals with the borders outside of a city which one may not go past on Shabbat.
In order to understand Eruvin it is helpful to understand how a typical city was structured in the mishnaic period. Houses shared a courtyard, each house opening up to a courtyard. Alleys connected several courtyards to the public areas, and there were two different types of alleys, a closed one (mavoy satum) and an open one (mavoy mefulash). A closed alley opened to the public domain on only one side, whereas an open alley was open on both sides.
Our tractate will deal with two different types of eruvin. Literally eruv means to mix something. This meaning will become apparent as we learn what an eruv is. According to Torah law (deoraita) it is permitted to carry something out of a house and bring it into a courtyard and vice versa. Similarly, one may, according to Torah law, carry something from a courtyard into a closed alley, provided that the open end of the alley had either a sidepost (lehi) or a beam (korah) over its top. The pole and the beam serve symbolically to close off the alley such that it is in reality closed from three sides and symbolically closed from the fourth.
The sages feared that if they allowed carrying from the courtyard or closed alley or house into one of the other of these areas (for instance from their house into the courtyard), people would forget and they would also carry into the public domain. Therefore they forbade this unless one set up an eruv. An eruv is a meal shared by all of the members of the courtyard, which turns the courtyard into a fictional private domain. Similarly, if they wanted to carry in the closed alley, all of the members of the alley had to share in a meal set up in one of the houses. The eruv for the courtyard is called the courtyard eruv (eruvei hatzerot) and the alley eruv is called alley partnerships (shitufei mevuot). Tractate Eruvin deals with the details of how these eruvin work.
The second (or really third) type of eruv is the border eruv (eruvei techumin). On Shabbat it is forbidden for a person to go more than 2000 cubits outside of the place where he is when Shabbat began. However, if he is in a city, he may travel freely anywhere within that city, no matter how large it is. The 2000 cubits are counted from outside of the city. This halakhah is learned from Exodus 16:29-30, Let everyone remain where he is; let no one leave his place on the seventh day. So the people remained inactive on the seventh day. His place is defined by the rabbis as his city, and not merely his house. The border of two thousand cubits is derived midrashically from Numbers 35:5, a verse which deals with the fields surrounding the Levite cities.
If a person needs to go beyond the Shabbat border for the purposes of a commandment, for instance to learn Torah or for a circumcision, the rabbis allowed him to set up an eruv that would allow him to extend that distance by another 2000 cubits. What he would do is put a meal someplace within his border and then from that point he could walk another 2000 cubits. If, for instance, he were to put his meal 2000 cubits outside the city, he could then walk 4000 cubits on Shabbat. However, if he does so, on the other side of the city he can not walk out at al. The 2000 cubits on the one side means that his border is at that point and therefore he cant walk out at all on the other side of the city. However, if he left it 1000 cubits outside of the city on one side, he could then walk 3000 cubits on that side and 1000 cubits on the other side. Our tractate deals extensively with the details these laws as well.
Tractate Eruvin is known to be a detailed and difficult tractate. I wish you luck in learning and a yasher koach for all of your continuing effort.