How do I correct subject-verb agreement in 'One of the boys are absent'?

The verb must agree with the real subject, not with the nearest noun. In 'One of the boys are absent', the true subject is 'One', which is singular, so the verb must be singular: 'is'. The phrase 'of the boys' is just a modifier describing which one, and the plural 'boys' inside it does not control the verb. The correct sentence is: One of the boys is absent. This pattern, 'one of the + plural noun', always takes a singular verb. The same applies to 'each of', 'every one of' and 'neither of', which are all singular. A reliable trick: mentally cover the 'of the...' phrase and check whether the remaining subject is singular or plural, then choose the verb accordingly.