“I'm sorry. I don't set the rules; I just enforce them.”
You work at a nightclub. The club has a strict dress code. A group of people want to get in, but one of them is wearing shorts. You can't let them in because of the club's rules. One of the people in the group is asking you to make an exception. You say this because you're not allowed to make this decision.
I'm sorry. I don't set the rules; I just enforce them.
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To "set the rules" for something means to decide what the rules are going to be:
These days, the power of the Internet is empowering consumers to set the rules.
To "enforce" a rule means to make sure that people follow it. People who "enforce" rules include police officers, teachers, HR staff at a company, store employees, and so on.
This kind of sentence is common when you're making a distinction between two related ideas. In the example above, the bouncer says:
I don't set the rules; I just enforce them.
This is something that employees of a store or company will sometimes say when people ask them to change a rule that they're not allowed to change. Some other examples include:
I don't hate him; I just don't want to talk to him right now.
I don't force him to take lessons; I just encourage him to stick with the things that he likes to do.
When you pronounce these, place the most stress on the verb in the first part of the sentence:
I don't force him to take lessons; I just...