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Ngữ pháp
Modality A2 B1

Obligation and advice

Express obligation, necessity, lack of necessity, prohibition, advice, expectation, and warning with have to, must, need to, should, ought to, and had better.

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Obligation and advice: the pressure dial

PRESSURE DIAL
↑ more pressure / less choice

BLOCKED: must not = no permission
  You must not park here.

RULE / NECESSITY: have to / need to / must
  You have to wear a helmet on the site.

WARNING: had better = strong advice + risk

ADVICE / EXPECTED RESULT: should / ought to
  You should take a jacket.
  The package should arrive tomorrow.

NO NEED: don't have to / don't need to
  You don't have to answer every question.

Key negative contrast:
  don't have to = no need
  must not = no permission
↓ less pressure / more choice

Choose the helper by how much pressure there is: no need, good idea, warning, rule, or blocked action.

Introduction

Choice, pressure, and advice

Obligation and advice tell the listener how much choice there is. English does not use one word for every situation. Use have to or must when something is required, and use need to when something is necessary for a result. Use should or ought to for advice, and use had better for urgent advice with a warning feeling. The main action stays in its base form inside these frames: You have to wear a helmet on the site. You should take a jacket.
Core Ngữ pháp A2 Từ vựng A2 Neutral

You have to wear a helmet on the site.

Have to expresses a real requirement, often from a rule or situation.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

have to wear
modal_obligation
Shows a required action with have to + base verb.
Học như một cụm cố định
a helmet on the site
context
Names what is required and where.
Core Ngữ pháp A2 Từ vựng A1 Neutral

You should take a jacket.

Should gives advice without making it a rule.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

should take
modal_advice
Gives advice with should + base verb.
Học như một cụm cố định
a jacket
object
Names the recommended thing.
Cognitive metaphor

A pressure dial

Think of these expressions as a pressure dial. Must and have to are high pressure: a rule, duty, or strong demand leaves little choice. Need to is also strong, but the pressure comes from a goal or practical result. Should and ought to are middle pressure: they point to the best, fairest, or expected path. Had better is a warning light: the choice still exists, but ignoring the advice may cause a problem soon.
Core pattern

Core frames

Build the sentence around a fixed frame. Have to and need to change with the subject: I have to, she has to, we need to, he needs to. Must, should, ought to, and had better do not take -s. The main action stays plain: She has to work late tonight. Questions with have to and need to use do: Do we have to pay now? Does he need to bring ID? Questions with should do not use do: Should I call him now? Do not add -s to the main verb: not She has to works.
Core Ngữ pháp A2 Từ vựng A2 Neutral

She has to work late tonight.

Have to changes to has to with she, but the main verb stays in the base form.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

She has to work
subject_obligation
Shows has to for he, she, or it, followed by the base verb work.
Học như một cụm cố định
late tonight
time
Adds when the obligation happens.
Question Ngữ pháp A2 Từ vựng A1 Neutral

Do we have to pay now?

Questions with have to use do or does.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

Do we have to pay
obligation_question
Forms a question with do + subject + have to + base verb.
Học như một cụm cố định
now
time
Adds the time of the required action.
Question Ngữ pháp A2 Từ vựng A2 Neutral

Does he need to bring ID?

Need to also uses do or does in questions.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

Does he need to bring
necessity_question
Forms a question with does + subject + need to + base verb.
Học như một cụm cố định
ID
object
Names what may be necessary.
Question Ngữ pháp A2 Từ vựng A1 Neutral

Should I call him now?

Should does not use do in questions.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

Should I call
advice_question
Asks for advice with should + subject + base verb.
Học như một cụm cố định
him now
object_time
Completes the action and time.
Contrast

Must, have to, and need to

Must and have to are the closest common confusion. Both can mean obligation, but have to is the normal everyday choice for outside facts, arrangements, and duties: I have to get up at six for work. Must often sounds stronger, more personal, or more official: Visitors must sign in. Need to focuses on necessity for a purpose, not authority: We need to charge the laptop before the meeting. Must I sign both copies? is correct but formal or serious. Do I have to sign both copies? is more common.
Core Ngữ pháp A2 Từ vựng A1 Neutral

I have to get up at six for work.

Have to is the usual choice for daily duties and outside facts.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

have to get up
external_obligation
Shows an everyday obligation caused by a schedule or situation.
Học như một cụm cố định
at six for work
time_reason
Gives the time and reason for the obligation.
Formal Ngữ pháp A2 Từ vựng A2 Formal

Visitors must sign in.

Must is common in signs, notices, instructions, and official rules.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

must sign in
official_rule
Shows an official rule with must + base verb.
Học như một cụm cố định
Core Ngữ pháp A2 Từ vựng A2 Neutral

We need to charge the laptop before the meeting.

Need to often focuses on what is necessary for a goal.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

need to charge
practical_necessity
Shows necessity for a practical result.
Học như một cụm cố định
the laptop
object
Names the thing affected by the action.
before the meeting
purpose_time
Explains why or by when it is necessary.
Formal Ngữ pháp B1 Từ vựng A2 Formal

Must I sign both copies?

Must questions are correct but often formal, official, or serious.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

Must I sign
formal_obligation_question
Asks about obligation with must; this sounds formal or serious.
Học như một cụm cố định
both copies
object
Names what may be required.
Contrast Ngữ pháp A2 Từ vựng A2 Neutral

Do I have to sign both copies?

This is more common in everyday speech than Must I sign both copies?

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

Do I have to sign
common_obligation_question
The everyday way to ask whether something is required.
Học như một cụm cố định
both copies
object
Names what may be required.
Usage rules

Negatives: no need or no permission

Negative forms change the meaning sharply. Don't have to and don't need to mean there is no necessity; the action is optional. You don't have to answer every question. He doesn't need to come to the meeting. Needn't has the same no-necessity meaning, but it is less common or more formal in everyday speech. Must not or mustn't means prohibition: You must not park here. Shouldn't means advice not to do something: You shouldn't text while driving. Do not confuse no necessity with no permission. If parking is forbidden, You don't have to park here is the wrong message; it means parking here is not required.
Negative Ngữ pháp A2 Từ vựng A2 Neutral

You don't have to answer every question.

Don't have to means no necessity, not prohibition.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

don't have to answer
no_necessity
Means the action is not required.
Học như một cụm cố định
every question
scope
Shows what is optional.
Negative Ngữ pháp A2 Từ vựng A1 Neutral

He doesn't need to come to the meeting.

Doesn't need to is another common way to say something is not necessary.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

doesn't need to come
no_necessity
Means coming is not necessary.
Học như một cụm cố định
to the meeting
destination
Names the event.
Negative Ngữ pháp A2 Từ vựng A1 Formal

You must not park here.

Must not means prohibition, not lack of necessity.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

must not park
prohibition
Means parking is forbidden.
Học như một cụm cố định
here
place
Gives the place where the rule applies.
Negative Ngữ pháp A2 Từ vựng A2 Neutral

You shouldn't text while driving.

Shouldn't gives advice against an action; it is usually softer than must not.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

shouldn't text
negative_advice
Gives advice not to do something.
Học như một cụm cố định
while driving
time_condition
Says when the advice applies.
Mistake Ngữ pháp A2 Từ vựng A1 Neutral

You don't have to park here.

This is the wrong message if you mean parking is forbidden; it means parking here is not required.

Corrected Ngữ pháp A2 Từ vựng A1 Formal

You must not park here.

Use must not, can't, or not allowed to for prohibition.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

must not park
prohibition
Correctly expresses that parking is forbidden.
Học như một cụm cố định
here
place
Gives the place where the rule applies.
Contrast

Should and ought to

Should is the normal verb for advice. Ought to means almost the same thing, but it is less common and can sound a little formal or moral: You ought to apologize. Use should + base verb, but ought to + base verb. The negative shouldn't is common; ought not to is correct but less frequent. Questions with should are normal, while question forms with ought to are formal. Should and ought to can also express expectation, not advice: The package should arrive tomorrow. She ought to be home by now.
Formal Ngữ pháp B1 Từ vựng A2 Neutral

You ought to apologize.

Ought to is close to should, but it is less common and can sound more formal.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

ought to apologize
moral_advice
Gives advice with a slightly formal or moral tone.
Học như một cụm cố định
Core Ngữ pháp B1 Từ vựng A2 Neutral

The package should arrive tomorrow.

Should can express expectation when the speaker thinks something is likely.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

should arrive
expected_result
Shows an expected result, not advice.
Học như một cụm cố định
tomorrow
time
Adds when the result is expected.
Core Ngữ pháp B1 Từ vựng A2 Neutral

She ought to be home by now.

Ought to can also express expectation, especially when something seems reasonable or normal.

Các thành phần cấu tạo

Đọc câu theo từng cụm, không đọc từng từ.

ought to be home
expected_situation
Shows an expected situation with ought to.
Học như một cụm cố định
by now
time
Means before or at this time.
Danger zone

Had better is a warning

Had better + base verb gives urgent advice with a possible bad result if it is ignored. It is usually about a specific situation now or soon, not a general habit: You'd better save the file now. The negative is had better not: You'd better not be late. The had is often shortened to 'd and does not make the meaning past. Do not add to after better. Wrong: You had better to save the file now. Correct: You had better save the file now. Questions with had better are uncommon; use should when you simply want advice.
Summary

Function map

Choose the expression by function. For strong obligation, use have to for most everyday duties and must for strong, official, or strongly stressed rules. For practical necessity, use need to. For no necessity, use don't have to or don't need to. For prohibition, use must not, can't, or not allowed to. For advice or expectation, use should; choose ought to when a slightly formal tone is acceptable. For urgent warning advice, use had better, especially with I, we, or close relationships.
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