Shabbos, chapter twelve, Mishnah 1. This Mishnah deals with the melachah of boneh (building) and with the melachah of makeh b'patish - delivering the final hammer blow to an object.
"HaBoneh, kamah yivneh viyhei chayav? HaBoneh kol shehu": - One who builds, how much must he build in order to be liable? One who builds any amount at all.
The Mishnah asks what the minimum measure is that renders a person liable for the melachah of building, and it answers that even one who builds the smallest amount is liable. This includes even filling in a small hole in a wall - the very act of filling it is considered building.
"V'hamesatet, v'hamakeh b'patish uv'matzad, hakodeach kol shehu - chayav": - One who chisels, one who strikes with a hammer or with a matzad, and one who drills any amount at all - is liable.
"V'hamesatet" - one who takes a stone and carves it in a way that smooths it.
"V'hamakeh b'patish" - one who delivers the final hammer blow to the object.
"Uv'matzad" - one who strikes with a matzad, which is a type of axe.
"Hakodeach kol shehu" - one who drills a hole, even the smallest amount.
All of these are various forms of makeh b'patish or of building, and for all of them one is liable even for the smallest measure.
"Zeh hak'lal": - This is the rule.
"Kol ha'oseh melachah um'lachto miskayemes b'Shabbos - chayav" - any melachah performed on Shabbos whose result endures, meaning that nothing needs to be added to it after Shabbos but it is complete as it stands, brings the work to completion. Therefore the one who does it is liable for makeh b'patish, whose essence is the finishing touch; and one who applies the finishing touch on Shabbos is liable.
The view of Rabban Shimon ben Gamliel:
Rabban Shimon ben Gamliel says: "Af hamakeh b'kurnas al hasadan bish'as melachah - chayav, mipnei shehu k'metaken melachah" - even one who strikes the anvil with a mallet in the course of his work is liable, because he is like one who is preparing the work.
What is this referring to? In the construction of the Mishkan they would flatten out a sheet of metal, and since they had no large presses, they did this by hand with a hammer and a smooth anvil. Keeping the hammer smooth was essential, since even the slightest imperfection could puncture the thin sheet. Therefore, every few blows to the sheet they would strike the hammer against the anvil in order to make sure that it remained smooth.
This act is makeh b'patish in the most literal sense - an actual blow with a hammer. It is not, however, a blow in the sense of the final hammer stroke, but rather, in the words of the Mishnah, in the sense of his being "like one who is preparing the work": readying the hammer and restoring it to its proper form, so that it can flatten the sheet without puncturing it.
In summary: In this Mishnah we learned that one who builds any amount at all is liable, and likewise one who chisels, one who strikes with a hammer or with a matzad, and one who drills any amount at all. This is the rule: anyone who performs a melachah on Shabbos whose result endures is liable. And Rabban Shimon ben Gamliel added that even one who strikes the anvil with a mallet in the course of his work is liable, because he is like one who is preparing the work.