Using Imperative, Permission, Obligation & Prohibition

Using the Imperative Form in English

You can use the imperative form to give an order, to give a warning or advice, and (if you use "please") to make a request.

To make the imperative, use the infinitive of the verb without 'to':
"Come here!"
"Sit down!"
To make a negative imperative, put "do not" or "don't" before the verb:
"Don't go!"
"Do not walk on the grass."
The imperative can be used for all subjects (you, he, they and we), but you can also use "let's" before the verb if you are including yourself in the imperative:
"Let's stop now."
"Let's have some lunch."
The negative of "let's" is "let's not":
"Let's not argue!
"Let's not tell her about it."
Orders
Adults do not usually give each other orders, unless they are in a position of authority. The intonation of an order is important: each word is stressed, and the tone falls at the end of the sentence:
"Sit down now!" ('Sit', 'down' and 'now' are all stressed, and the tone falls on 'now'.)
However, adults can give orders to children and to animals:
Here are some orders you could give your pet dog:
"Come here!"
"Sit!"
"Heel!"
"Fetch!"
Warnings
You can use the imperative to warn someone of danger. All the words in the warning are stressed, but the last word has a higher tone than the first word:
"Watch out!"
"Look out!"
"Don't cross!"
Advice
When you give advice using the imperative, the words are stressed normally:
"Eat an apple – it's much better for you than a biscuit!"
"Don't tell him you're resigning now! Wait until Monday when he's in a better mood."
You can often read articles in magazines that give advice on a subject. Sometimes, this advice is presented as "Dos and don'ts".
For example:
Travelling long-distance
Do try to sleep well the night before
Do drink plenty of water
Do try to walk about the plane during the flight
Don't drink alcohol
Don't eat heavy meals
Don't wear restrictive clothing
Requests
You can also use the imperative to make a request, but you should use a polite word before the verb:
"Please take a seat."
"Please wait here."
"Please hold the line."
"Please don't smoke here."
In written English, you might also see "Kindly" used as a polite word:
"Kindly return the documents as soon as possible."
"Kindly forward this to the Sales and Marketing department."
"Kindly send me 2 copies of your brochure."

Source: http://www.english-at-home.com/grammar/imperative-form/

Common English Words: Permission, Obligation, Prohibition

Permission = It’s OK

You can smoke here

English words used for permission:

§  can

§  allowed

§  may

§  permitted

What’s the difference? 

The word “to” is used after allowed and permitted, but not after can or may.

§  You’re allowed to smoke in here.

§  You can to smoke in here.

§  You can smoke in here.

Can is more informal, may and permitted are more formal, and allowed is both formal and informal.

For the past and future of “can,” you can use could / was allowed (in the past) and will be allowed (in the future):

§  When I was a kid, I was allowed to stay up until 11 PM on Friday nights.

§  We‘ll be allowed to check two suitcases on the flight.

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Obligation = It’s necessary

You have to show your photo ID.

English words used for obligation:

§  have to

§  need to

§  must

§  required

What’s the difference?

Must and required are more formal than have to and need to.

Don’t use “to” after “must.”

Supposed to / Not supposed to

You can use these words for “light” requirements and prohibitions – for example, rules that are often not followed.

§  We’re supposed to arrive on time, but it’s OK if we’re a little late. Our boss is pretty relaxed.

§  The employees aren’t supposed to eat lunch at their desks, but a lot of them do so anyway.

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Prohibition = It’s not OK

You’re not allowed to swim here.

English words used for prohibition:

§  can’t

§  mustn’t

§  not allowed

§  not permitted

What’s the difference?

Can’t is more informal, mustn’t and not permitted are more formal, and not allowed is both formal and informal.

Don’t use “to” after “mustn’t.”

Pronunciation

The first “t” in “mustn’t” is silent!

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No obligation = It’s not necessary

You don’t need to wear shoes here.

English words used for no obligation:

§  don’t have to

§  don’t need to

§  not necessary

§  not required

§  optional

What’s the difference?

Don’t have to and don’t need to are more informal. Not necessary, not required, and optional are more formal.

 

Source: http://www.espressoenglish.net/common-english-words-permission-obligation-prohibition/