1.

Rashi: It is a spirit of jealousy (from Above) that prompts the husband to warn his wife not to seclude herself with the man involved.

2.

Sotah, 3a: "Kin'ah" has connotations of purity, and we learn from here that a man only warns his wife when a spirit of purity ¹ enters into him.

1.

Rashi: It means that he warns ¹ his wife not to seclude herself with the man whom he suspects of having an affair with her. ²

2.

Sotah, 3a #1: It means that, if a man suspects his wife of having an affair with another man, he is obligated to warn her ³ - which overrides the La'av of "Lo Sisna es Achicha bi"Lebavecha". ⁴

1.

Rashi #1: It means that she secluded herself with the man long enough to have been intimate with him.

2.

Rashi #2 (in Yevamos, 58a): It means that since she secluded herself with the man, she becomes forbidden to her husband as if she commited arultery ? even though we don't know that she did.

1.

Rashi and Rashbam: It implies that he warned his wife not to seclude herself with the man, and she did, but he does not know whether they were intimate or not. ¹

2.

Sotah, 28a: If she is Tamei, why does she drink" If she is not Tamei, why does the Kohen give her to drink? The Torah must therefore be telling us that although she is only a Safek, she is forbidden to her husband until she drinks the water - and the Safek is resolved. ²

1 Seforno: The first time refers to where he warned her because he was imbued with a spirit of jealousy, when he realized that she had departed from the path of Tz'niyus; the second time, where he warned her for no apparent reason (and either way, she contravened his warning).

1.

Sotah, 28a: To teach us that the woman is forbidden to her husband, and to the adulterer ¹ and that, if she is the wife or the daughter of a Kohen, she is forbidden to eat Terumah.