chapter  17
The modal auxiliaries
Pages 17

They are called ‘modal’ verbs because they indicate the attitude of the speaker with regard to what is being said, expressing ideas like ability, possibility, permission, necessity, obligation and volition. This meaning is very similar to that of the modal particles (see Chapter 10) and the category of mood (see Chapter 16). They are referred to as ‘auxiliary’ verbs because they are mainly used with other verbs.