The definition of circumstances is: "The time, place, occasion, or surroundings of an act or event." or "Fact, occurrence, or condition."
It means the situation you are in, ie. If the weather is agreable and you have the right conditions, circumstances, the odds of getting a cab are less
You spelt it ok, if you have a dictionary you can look it up. Some thing that has or is going to happen. A situating "Under what circumstances"? Circumstance is a fact or condition with to or an event or fact that causes something to happen.
Circumstances are situation and environment.
The circumstances is your overall circumstances or condition in life.