Modal Verbs in English Grammar

Modal verbs, also known as auxiliary verbs, function as operators within clauses and help determine what action is being modified by another verb. These verbs have several key features:

• They can't stand alone without some other verb to modify; they always come before another verb, except in imperatives where they may come after it.
• Most modal verbs show aspect—a property of actions expressed through time phrases like yesterday or tomorrow.
• Some modal verbs indicate how often something happened; others tell you which tense was used.
• Many modal verbs are 'non-finite', meaning their past participle form has different meanings from its main verb counterpart. For example, was written means written previously, while written simply shows possession.
• There isn't much difference between the past forms of non-finite modals: all these forms take place in the past with little variation.

Modal verbs convey information about the action they modify, such as necessity, permission, obligation, possibility, etc., and they don't change their own meaning when combined with other verbs. However, combining two modals creates new meanings because each one adds extra nuances to the original thought. In English, there aren't many true modal verbs, compared to languages like French, but common ones include can, may, must, and others.