Obligation and prohibition — have to, must, shouldWhat is required, forbidden, and advisable.Related learning areasEnglish GrammarRelated conceptsAbility and possibility — can, could, be able toArticles — a, an, the, and no articleChoosing between comparatives and superlativesChoosing between conditionalsChoosing between gerunds and infinitivesFirst conditional and future time clausesQuestionsAll questions in this subjectStart learningStart free practice