פירוש על אהלות 18:7

הקונה שדה בסוריא – Aram Naharayim and Aram Tzoba that David conquered. But its dust is impure like what is outside of the Land of Israel, but one is liable for tithes and Seventh Year produce like the Land of Israel (see also Tractate Demai, Chapter 6, Mishnah 11 and Second Samuel, Chapter 10 and Second Chronicles, Chapter 19 and Tractate Gittin folio 8a).

If one buys a field in Syria near to the land of Israel:
If he can enter it in cleanness, it is deemed clean and is subject to [the laws of] tithes and sheviit [produce];
But he cannot enter it in cleanness, it [is deemed] unclean, but it is still subject to [the laws of] tithes and sheviit [produce].
The dwelling-places of non-Jews are unclean. How long must [the non-Jew] have dwelt in [the dwelling-places] for them to require examination? Forty days, even if there was no woman with him. If, however, a slave or [an Israelite] woman watched over [the dwelling-place], it does not require examination.
Section one: The land of Syria (this is not the same as modern day Syria, but the general area is probably close) is impure as are all lands outside of Israel.
If one buys a field in Syria that lies right next to the border with Israel, and he can enter there in cleanness, meaning that there is no cemetery on the Israeli side, then the field on the Syrian side is considered clean. In other words, even though this land is not technically in Israel, since it was bought by a Jew and he can enter there without otherwise being defiled, it counts as part of the land of Israel (definitely not getting into any political implications of this mishnah).
Any produce grown on the land is liable for tithes and is subject to the laws of sheviit (the sabbatical year).
Section two: However, if he cannot enter there while maintaining his purity because he would have to go through a cemetery, then the field itself is unclean. It is still subject to the laws of tithes and sheviit, because the land of Syria is always subject to these laws.
Section three: The rabbis believed that non-Jews would bury their miscarriages inside their houses. I don't know if this is historically true, but it is clear that the rabbis believed this. Therefore, any home owned by a non-Jew, even inside the land of Israel, needed to be examined to see if the non-Jew had buried a fetus there, provided that the non-Jew lived there for at least 40 days. The house needs to be examined even if he doesn't seem to be living with his wife, lest he brought another woman into his house.
If an Israelite, even a slave or a woman, was watching over the house so that he wouldn't bury a miscarriage inside the house, then the house is pure. In other words, the non-Jew doesn't inherently defile the house by his presence. For the house to be defiled there must be the possibility that he buried a miscarriage there.

אם יכול להכנס לה בטהרה – for the Land of the Nations does not form a partition between that field that he bought to the Land of Israel.

בטהרה – that field. And even on its own clod of earth they did not decree, because it belongs to an Israelite, and because of this, it is taught [in the Mishnah]: "הקונה שדה"/a person who purchases a field. But if it had the width of a handbreadth from the Land of the Nations or from the cemetery, it interrupts between the Land of Israel and a field that he purchased in Syria, for now, he is not able to enter in ritual purity, the soil of that field is impure like outside of the Land of Israel.

מדורות הגוים (Gentile dwelling places) – that are in the Land of Israel.

טמאים – they bury their aborted fetuses in their homes.

ארבעים יום – like the measure of the creation of the embryo.

אע"פ שאין עמו אשה – lawless/unrestrained men in the towns and they bring women to their homes at night.

עבד – [slave] of an Israelite (i.e., a Canaanite slave that is in the hand of an Israelite – according to the commentary of Tiferet Yisrael).

או אשה – an Israelite [woman].

משמרין – the Gentile dwelling places, that they (i.e., Israelites) will not be buried there.