SHOULD (NT) (SUGGESTIONS)
1: They don't use as for the third person singular. 2: They make questions by inversion ('she can go' becomes 'can she go?') 3: They are followed directly by the infinitive of another verb (without 'to')
We use should and shouldn't to give advice or to talk about what we think is right or wrong. You should means something like I think it is a good idea for you to do it. You shouldn't means something like I think it is a bad idea for you to do it. Should is used to express the opinion of a speaker and often follows I thinkor I don't think.
Examples
You look tired. I think you should take a few days off. Alice works very long hours. She should to talk to her boss. - I have an English test tomorrow. - I shouldn't worry if I were you. You have worked really hard. - I never have enough money. - I don't think you should go out so much.
MUST (OBLIGATION)
"Must" is most commonly used to express certainty. It can also be used to express necessity or strong recommendation, although native speakers prefer the more flexible form "have to." "Must not" can be used to prohibit actions, but this sounds very severe; speakers prefer to use softer modal verbs such as "should not" or "ought not" to dissuade rather than prohibit.
Examples:
This must be the right address! Certainty
Students must pass an entrance examination to study at this school. Necessity
You must take some medicine for that cough. strong recommendation
Jenny, you must not play in the street! prohibition
Modal Use
1. = Present 2. = Past 3. = Future 1. That must be Jerry. They said he was tall with bright red hair. 2. That must have been the right restaurant. There are no other restaurants on this street. 3. NO FUTURE FORM
HAVE TO (NECESSITY)
"Have to" is used to express certainty, necessity, and obligation.
Examples: This answer has to be correct. certainty
The soup has to be stirred continuously to prevent burning. Necessity They have to leave early. obligation
Use
1. = Present 2. = Past 3. = Future 1. That has to be Jerry. They said he was tall with bright red hair.
2. That has to have been the right restaurant. There were no other restaurants on the street. 3. NONE
DON'T HAVE TO (NO OBLIGATION)
Don't have to expresses a lack of obligation: it's equivalent to Don't need to or Needn't. There is no obligation.
Examples:
You don't have to eat. You doo brush your hair every ten minutes, darling.
MUST NOT, MUSTN'T (PROHIBITION)
We use must not to say that something is not permitted or allowed, for example:
Passengers must not talk to the driver.
Structure of Must not
Must is an auxiliary verb. It is followed by a main verb. The structure for must not is: subject + must not + main verb The main verb is the base verb (infinitive without "to"). Must not is often contracted to mustn't. Look at these examples: SUBJECT AUXILIARY MUST + NOT MAIN VERB I mustn't forget my keys. You mustn't disturb him. Students must not be late.
NB: like all auxiliary verbs, must CANNOT be followed by "to". So, we say:
You mustn't arrive late. (not You mustn't to arrive late.)
Use of Must not
Must not expresses prohibition - something that is not permitted, not allowed. The prohibition can be subjective (the speaker's opinion) or objective (a real law or rule). Look at these examples:
I mustn't eat so much sugar. (subjective) You mustn't watch so much television. (subjective) Students must not leave bicycles here. (objective) Policemen must not drink on duty. (objective) We can use must not to talk about the present or the future:
Visitors must not smoke. (present) I mustn't forget Tara's birthday. (future)
We cannot use must not to talk about the past. We use other structures to talk about the past, for example:
We were not allowed to enter. I couldn't park outside the shop.
