Sotah, Chapter 1, Mishnah 2. Having learned the basic law of the sotah, the Mishnah now sets out to clarify what "how does he warn her" means - in what wording the warning of jealousy is delivered, and how the law changes according to the formulation the husband used.
A warning about speaking alone:
"Amar lah bifnei shnayim: al tedabri im ish peloni" - the husband tells her in front of two witnesses not to speak with a certain man. There is no mention here of secluding herself with him, only a prohibition against speaking. "Vedibrah imo" - and she transgressed his warning and spoke with him. In such a case: "Adayin hi muteres l'veisah u'muteres le'echol biTerumah" - she remains permitted to her husband ("her house" means her husband), and she may even continue to eat Terumah.
Implicit here is a principle that has not yet been mentioned: a woman who has the status of a sotah, as long as she has not drunk the sotah waters, carries a concern that she may in fact have become defiled and had forbidden relations. And a woman who had forbidden relations - the wife of a kohen or the daughter of a kohen - may not eat Terumah. In our case, though, she remains permitted to eat Terumah, because there was no warning of jealousy here at all: merely speaking with a man is not the warning the Torah has in mind, for the warning concerns secluding herself with him.
At this point a few words need to be added into the Mishnah, to point out that although the Mishnah mentioned only that she spoke with him, in fact even if she secluded herself with him she would not become forbidden - because the warning was about speaking alone, and the husband never warned her not to seclude herself with him, so there is no warning of jealousy of any kind here.
A warning about seclusion:
Another line to be inserted into the Mishnah: had he told her "do not seclude yourself with a certain man," the outcome depends on what she then did. If, following this warning, she only spoke with the man, it is obvious that she does not become forbidden. But here the Mishnah continues: "Nichnesah imo l'veis haseser v'shahasah imo k'dei tumah" - after being warned not to seclude herself with him, she entered a place of concealment with him, a private house, and remained there long enough for marital relations to take place, which is what is called "tumah" here. In such a case: "Asurah l'veisah va'asurah le'echol biTerumah" - she is forbidden to her husband and forbidden to eat Terumah, until she drinks the sotah waters, which will determine whether she is guilty or innocent.
If the husband died without children:
Suppose the husband died before he managed to have her drink the sotah waters, and he had no children - in which case she would ordinarily come before her late husband's brother for yibum. In this case the brother cannot perform yibum with her; he only performs chalitzah. The reason: regarding a woman who had forbidden relations, or who is at least suspected of it, the Torah says that she leaves his house, is divorced, and marries another man, and from this the Sages derive that she may marry another man but not the brother-in-law. Nevertheless she does require chalitzah, for just as she would have required a get had her husband been alive, even if she had had forbidden relations, so too she requires chalitzah from the brother-in-law.
In summary: in this Mishnah we learned that the wording of the warning is what determines the law. A warning about speaking alone is not a warning of jealousy, and therefore even if she spoke with him - and even if she secluded herself with him - she does not become forbidden to her husband or to Terumah. A warning about seclusion, however, after which she entered a place of concealment with him and remained long enough for relations, renders her forbidden to her husband and to Terumah until she drinks the sotah waters. We also learned that if the husband died without children, the brother-in-law does not perform yibum with her but only chalitzah.