How do you choose the correct word in a sentence that has a fraction as its subject?
Fractions such as one-half and one-third may be singular or plural, depending on the word to which the fraction refers.
How do I decide?
When the subject is a fraction, amount of money, measurement, weight, volume, or interval of time, use a singular verb when referring to a single unit (one half of the cake is gone) and a plural verb when referring to the number of separate units (one half of the team members are absent).
Half of the mail (has, have) been opened.
Answer: Half of the mail has been opened.
Half of the letters (has, have) been opened.
Answer: Half of the letters have been opened.
Two-thirds of his hair (is, are) gone.
Answer: Two-thirds of his hair is gone.
Note: this rule is an exception to subject-verb agreement rules: normally, we do not make a word inside a prepositional phrase agree with the verb!