Yichud

Basics

1. It is forbidden from the Torah for a man to seclude himself with a women to whom he is forbidden. It is Rabbinically forbidden for a man to seclude himself with even a single woman. It is also Rabbinically forbidden for a man to seclude himself with a non-Jewish woman.
2. It is permitted for a man to seclude himself with his mother, daughter, granddaughter, or wife even when she in her state of Niddah. It is permissible for a man to seclude himself with his sister, however, it shouldn't be done frequently.
3. If a woman's husband is in the city, it's permissible to seclude with her unless one has a close relationship with her such as if one grew up with her or is a relative.
4. According to Sephardim, one may not be secluded with multiple woman nor one woman may not be secluded with multiple men.

Car

1. A man and woman may not drive together in a car on a road that's not heavily traveled especially at night. However, if there's three men and three women on the bus it's permissible. However, if some of the men and women leave and one woman is alone on the bus with male driver the woman must get off the bus unless there is a danger to get off. However, if there's a car on that road every 3 minutes it's permissible.

Elevator

1. It is permissible for a man to enter an elevator in which there is a woman if the elevator has a travel time of less than 3 minutes.

Different rooms

1. Many authorities hold that one man may not be in the same house as a woman even if they are in different rooms.