The word "rot" is used when something organic (alive) begins the process of decay. It’s similar to the word "spoil." When "rot" a verb, it looks like this:
simple
past
past participle
rot
rotted
rotted
Apples rot quickly when they fall from a tree.
When food rots, you have to throw it away.
Meat will rot if you don’t keep it refrigerated or frozen.
We watered the plant too often and the roots rotted.
Matt’s teeth are rotting because he doesn’t brush them regularly.
When wood begins to rot, it becomes an attractive home for insects.
Your teeth will rot if you don’t take care of them.
You can also use "rot" as a noun:
There’s some rot on this apple.
Signs of rot on the outside of a wooden house should be addressed immediately.
The green rot on old bread is called mold.
As an adjective, you can use "rotten," "rotted," or "rotting."
When meat becomes rotten it really stinks.
There’s nothing worse than the smell of rotting meat.
Rotted banana peels outside the house indicate the owners don’t clean up after themselves.
That was a really rotten thing that he did to his girlfriend. (Behavior is often described as "rotten" if it’s really bad.)
This is a rotten situation. (It’s a bad situation.)
Your teeth will rot if you don’t take care of them.