how old do you have to be — have to + bare be (GiU Unit 31)
The frame
Section titled “The frame”How old do you have to be to have a driving licence? ✓
Words: do + you + have to + be (given tokens be / you / to / have / do).
Derivation
Section titled “Derivation”- Statement with gap: You have to be [17] to have a licence.
- Ask about the gap: front how old + do-support → How old do you have to be [__] to have a licence?
How old is the wh-phrase that fills the complement slot after be. be is the main verb (the question asks about a state = an age, not an action).
Why be, not another verb
Section titled “Why be, not another verb”have to + bare verb. The verb is be because the answer is a state:
- How old do you have to be? → “17” (state)
- How much do you have to pay? → “$20” (action)
Trap 1 — dropping have to
Section titled “Trap 1 — dropping have to”✗ You are 17 to have a licence. — plain be can’t carry the purpose infinitive; the obligation frame is needed.
Trap 2 — be to + infinitive (different idiom)
Section titled “Trap 2 — be to + infinitive (different idiom)”be + to + infinitive = formal official instruction (= must, orders/signs):
- You are to report at dawn.
- Passengers are to remain seated.
- You are to have a licence = you must obtain one (nothing about age).
JP mirror
Section titled “JP mirror”何歳でなければならないか = how old do you have to be (〜なければならない = have to).
Source
Section titled “Source”Raymond Murphy, English Grammar in Use, Unit 31 — have to. Ebot lesson 2026-09-05.