Definition of should modal verb from the Oxford Advanced American Dictionary

      

    should

     modal verb
    modal verb
    NAmE//ʃəd//
     
    , NAmE//ʃʊd//
     
     
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  1. 1used to show what is right, appropriate, etc., especially when criticizing someone's actions You shouldn't drink and drive. He should have been more careful. A present for me? You shouldn't have! (= used to thank someone politely)
  2. 2 used to say that you expect something is true or will happen We should arrive before dark. I should have finished the book by Friday. The roads should be less crowded today.
  3. 3 used to say that something that was expected has not happened It should be snowing now, according to the weather forecast. The bus should have arrived ten minutes ago.
  4. 4 (formal) used to refer to a possible event or situation If you should change your mind, just let me know. In case you should need any help, here's my number. Should anyone call (= if anyone calls), please tell them I'm busy.
  5. 5 used after that when something is suggested or arranged She recommended that I should take some time off. This idea can be expressed without “should”:She recommended that I take some time off.
  6. 6 why, how, who, what should somebody/something do used to refuse something or to show that you are annoyed at a request; used to express surprise about an event or a situation Why should I help him? He's never done anything for me. How should I know where to find you? I got on the bus and who should be sitting in front of me but Tony!
  7. 7 used after that after many adjectives that describe feelings I'm anxious that we should allow plenty of time. I find it astonishing that he should be so rude to you.
  8. 8 used with I and we to give opinions that you are not certain about I should imagine it will take about three hours. “Is this enough food for everyone?” “I should think so.” “Will it matter?” “I shouldn't think so.”
  9. 9 used for expressing strong agreement “I know it's expensive but it will last for years.” “I should hope so too! “Nobody will oppose it.” “I should think not!
  10. 10 used to tell someone that something would amuse or surprise them if they saw or experienced it You should have seen her face when she found out! Grammarmodal verbsThe modal verbs are can, could, may, might, must, ought to, shall, should, will, and would. Dare, need, have to, and used to also share some of the features of modal verbs.Modal verbs have only one form. They have no past or present participles and do not add -s to the 3rd person singular form:He can speak three languages. She will try and visit tomorrow.Modal verbs are followed by the infinitive of another verb without to. The exceptions are ought to and used to:You must find a job. You ought to stop smoking. I used to smoke, but I quit two years ago.Questions are formed without do/does in the present or did in the past:Can I invite Mary? Should I have invited Mary?Negative sentences are formed with not or the short form -n’t and do not use do/does or did:You shouldn't invite Mary. The error will not have affected our results.You will find more help with how to use modal verbs at the dictionary entries for each verb.
Grammarshould / ought to / had betterShould and ought to are both used to say that something is the best thing or the right thing to do, but should is much more common:You should take the baby to the doctor’s. I ought to give up smoking.In questions, should is usually used instead of ought to:Should we call the doctor?Had better can also be used to say what is the best thing to do in a situation that is happening now:We’d better hurry or we’ll miss the train.You form the past by using should have or ought to have:She should have asked for some help. You ought to have been more careful.The forms should not or shouldn’t (and ought not to or oughtn’t to, which are rare) are used to say that something is a bad idea or the wrong thing to do:You shouldn’t drive so fast.The forms should not have or shouldn’t have and, much less frequently, ought not to have or oughtn’t to have are used to talk about the past:I’m sorry, I shouldn’t have lost my temper. Grammarshould / wouldWould is used in reported sentences, conditions, requests, etc. In spoken English, the short form 'd is usually used:I said I’d (I would) be late. He’d (he would) like to have been an actor. I’d (I would) really prefer coffee.The main use of should is to tell somebody what they ought to do, to give advice, or to add emphasis:We should really go and visit them soon. You should have seen it!
See the Oxford Advanced Learner's Dictionary entry: should