Most modals do not have singular and plural forms. They do not change their form, therefore they are the same in all three persons.  These modals are ‘could’, ‘may’, ‘might’, ‘must’, ‘shall’, ‘should’, ‘will’, ‘would’, and ‘ought to’. However, the modals ‘have to’ and ‘need to’ are exceptions and have to agree with their subject, whether singular or plural.