inform on (or inform against): to secretly give the police or someone in authority information about someone, Tommy would never have gone to jail if one of his crew hadn't informed on him to the cops

Note the difference between this and other combinations like "inform of" or "inform about", which are more innocuous. Another example:

Before the end of the Cold War many in the country were afraid to speak their political views openly, for fear that their neighbors would inform on them to the secret police.

It's a good idea to memorize English verb-preposition pairs individually, since together they can have a very different meaning from the verb by itself.

inform on (or inform against): to secretly give the police or someone in authority information about someone.

For example:

Tommy would never have gone to jail if one of his crew hadn't informed on him to the cops

Another example:

Before the end of the Cold War many in the country were afraid to speak their political views openly, for fear that their neighbors would inform on them to the secret police.

Note the difference between this and other combinations like "inform of" or "inform about", which are more innocuous. It's a good idea to memorize English verb-preposition pairs individually, since together they can have a very different meaning from the verb by itself.