A singular test is a SQL file under tests that returns the rows violating a business rule. dbt succeeds when that query returns zero rows. That direction is easy to reverse: selecting valid rows makes a healthy model fail.
Give the test a name that describes the invariant, then write its predicate as the exception.
Use singular tests for rules that don't fit a single column, such as a completed order without a known amount. Be explicit about NULL. A comparison like amount < 0 doesn't include missing amounts.
If the rule rejects both negative and unknown values, write both conditions. The tasks use models that satisfy the intended invariant so the test can pass, but the checker also requires the test file and its specific failing-row logic.