Final answer:
The sentences that use hyphens correctly are Mary is an ex-teacher from Lincoln High, Thirty-three students went to the tournament, and Tom liked to read about pre-Roman times.
Step-by-step explanation:
The sentences that use hyphens correctly are:
Mary is an ex-teacher from Lincoln High.
Thirty-three students went to the tournament.
Tom liked to read about pre-Roman times.
The use of hyphens in these sentences helps to clarify how multiple modifiers function before a noun. In sentence 1, 'ex-teacher' is a single modifier describing Mary. In sentence 2, 'Thirty-three' is a compound modifier describing the number of students. In sentence 3, 'pre-Roman' is a compound modifier describing the times that Tom likes to read about.