Form linked to queries

[ram]

I have a database to track monthly expenses.

I enter the following on Form A:
TxtBox1 Agent ID
TxtBox2 Rent Amount
TxtBox3 Rent Date

After I update txtBox3 I would like to have the system query the underlying
table to make sure nothing has already been entered for rent on this date.

I have been told I should be able to make a query based on the form to
perform this task.

Thanks again for any help