Mishnah/Seder Nashim/Tractate Sotah/Chapter 1/2
How do you warn her? If he says to her in front of two witnesses: Don't speak with so and so, a man, and she speaks with him - behold, she is still permitted to her house and to eat terumah.
If she enters with him into a hidden house, and is there for a sufficient time to be defiled - She is forbidden to her house and forbidden to eat Terumah.
And if he (the husband) dies - She must perform Chalitzah and may not perform Yibum.