if (something) is not (done), (something) will be (done)

In the situation above, the person writing the letter is explaining what consequences the tenant will face if he doesn't pay his rent by a certain date. This is a kind of threat. People can get really angry, frightened, or offended if they feel like they're being threatened in a direct way. For example, writing the sentence above this way would probably start an argument:

If you don't pay us by January 15th, we're going go charge you a $150 penalty.

This version is too direct for communicating with a customer or tenant. Instead, the person in the example explained the consequence of not paying in a more indirect way:

Here are some other examples:

If it isn't completed by the end of the month, someone is going to get fired.

If all the steps are not followed in the correct order, the results will be ruined.

This phrase appears in these lessons: