Bava Metzia, Chapter 9, Mishnah 10 - the final Mishnah in the series of mishnayos dealing with the rights and obligations of farmers and landowners. This Mishnah relies entirely on linguistic nuance, and since it is based on a precision in the Hebrew language that lacks an exact translation, it is difficult to understand from the Mishnah itself. Therefore, we will start with a parallel and understandable case, and from there we will return to the language of the Mishnah and see how things work.
A Parable: Renting a Printing Press:
Renting "for a week": A person who needs to print magazines rents a printing press for seven hundred dollars for an entire week. Since we live in the Land of Israel and both parties are Jewish, and understand that there is halachic sensitivity here, it is obvious that the week includes Shabbos. The renter gets the machine for seven days, six of which are work days and one is not a work day.
Renting for "seven production days": But if the renter said that he needs the machine for seven production days for seven hundred dollars, it is understood that Shabbos is not counted among them, since it is not a production day. In this case, he will receive seven actual work days, and Shabbos is not one of them.
From the Parable to the Lesson - the Shemittah Year:
This is the concept at the foundation of the Mishnah, but instead of days of the week, the Mishnah deals with the years of the seven-year cycle, which includes the Shemittah year. The Shemittah year is like Shabbos for the land: the land is not worked during it, and the produce of the land is not at man's disposal, at least not commercially.
From this come the two types of rentals in the Mishnah:
Renting out "for seven years": If the farmer agreed to a period of seven years - the equivalent of a "week" - for the price of seven hundred zuz, the Shemittah year is included in the period. Just as when one rents an olive press for a week, one day of the week is Shabbos.
Renting out "seven harvest years": But if the nature of the transaction is that he needs seven years of agricultural work for seven hundred zuz, it is understood that the Shemittah year is not included in the seven, because he was meant to receive seven years of crop yield, and the Shemittah year is not counted in this tally at all.
It is worth being precise: "Shevi'is" mentioned here does not mean the seventh year of the rental period. The intent is the Shemittah year within the seven-year cycle, and it can fall in the first of the rental years, in the middle of them, or in the last of them. The main point is that the Shemittah year is included in the "week", in the seven years.
In summary: In this Mishnah we learned that everything depends on the language of the agreement. For one who rents the land for a period of seven years - the Shemittah year is included in the years, just as Shabbos is included in the days of the week; whereas for one who rents it for seven years of work and harvest - the Shemittah year is not counted among them, just as Shabbos is not counted in the production days. With this, the mishnayos conclude their discussion of the rights and obligations between the farmer and the landowner.