Use the word "ruin" when the quality of something has decreased a lot. It can be a situation or a relationship that is suddenly very bad:
She ruined dinner by leaving the food in the oven for too long.
The fire ruined the house. (active voice)
The house was ruined by the fire. (passive voice)
Carl ruined the relationship he had with his girlfriend by cheating on her.
Bad weather ruined the party we planned to have outside.
The women talked nonstop during the movie and ruined it for everyone.
You’re ruining my plans.
My plans are ruined. (You can use "ruined" as an adjective.)
You can also use "ruin" as a noun. In this case, it’s something that is completely destroyed, or the passage of time has damaged something, such as a building.
Their house lay in ruins after the fire.
Their plans are in ruins because the city wouldn’t give them a license to do business.
When you visit Italy, you can see the ruins of the Roman empire.
The army that invaded the city left it in ruins.
The ruins you see in Greece are thousands of years old.
