eng8-u4-grammar-a· UNIT 4. MANNERS AND RULES· ~17 min

Unit 4 Grammar A: Obligation, prohibition, and no necessity

Use modal meaning, verb form, time, and question structure precisely.

Must and have to both express necessity. Mustn’t and cannot express prohibition: an action is against a stated rule. Don’t have to expresses absence of necessity; the action is optional, not forbidden. These forms are followed by the base verb: must enter, cannot take, have to wait. Have to changes for subject and time: she has to, she doesn’t have to, and everyone had to in the past. Questions use either Must + subject + base verb or Do/Does + subject + have to + base verb.

Key terms

necessity — something required or important to do
prohibition — a rule saying an action is not allowed
no necessity — an action is optional because it is not required
base verb — the unchanged verb form used after a modal, such as enter or wait
Meaning and form
MeaningFormsExample from a stated rule
Necessitymust / have toVisitors must sign in.
Prohibitionmustn’t / cannotVisitors cannot enter this room.
No necessitydon’t / doesn’t have toVisitors don’t have to book.
Past necessityhad toWe had to leave early.
Question patterns
Requested formPatternExample
mustMust + subject + base?Must we wait here?
have toDo/Does + subject + have to + base?Do we have to wait here?
✎Distinguish optional from prohibited
  1. 1Read the stated rule: Booking is optional on weekdays.
  2. 2Choose the meaning: Optional means there is no necessity.
  3. 3Choose the form: You don’t have to book on weekdays.
🚫Negative forms differ

You don’t have to enter means entry is optional; you mustn’t enter means entry is prohibited.

💡Use the stated context

A sign or rule question supplies the rule you need; do not assume a universal rule for every public place.

Rules

  1. 1Use must or have to to express necessity.
  2. 2Use mustn’t or cannot for a stated prohibition.
  3. 3Use don’t/doesn’t have to when an action is not necessary and remains optional.
  4. 4Do not confuse don’t have to with mustn’t: optional and prohibited have different meanings.
  5. 5Use a base verb after must, mustn’t, and cannot; do not add to.
  6. 6Use have to with I/you/we/they and has to with he/she/it.
  7. 7Use had to for obligation in the past.
  8. 8Form questions as Must + subject + base verb or Do/Does + subject + have to + base verb.

Practice

4 easy · 4 medium · 4 hard

Up to 10 random questions per test

EasyMediumHardMixed