Look at the following sentences.
- She will come.
- He can sing.
- She may win.
- You must wait.
- She should go.
These sentences are not statements of fact. They express actions or events that exist only as conceptions of the mind – probabilities, possibilities, obligations, wishes, expectations etc. Several factors may prevent these probabilities, possibilities etc., from being fulfilled. Auxiliaries which help to express such conceptions of the mind are called modal auxiliaries. Modal is the adjective of mode which means mood or manner.
Modal auxiliaries help to express the subjunctive and imperative moods. They may even be called colored auxiliaries because they are colored by the speaker’s feelings.
However, it must be admitted that modal auxiliaries do not always express such moods. Sometimes they may express simple futurity.
Study the following example.
I will turn eighteen on Monday. (Here we are expressing simple futurity without any hint of personal feelings.)
On the other hand, primary auxiliaries (be, do and have) may sometimes be used with modal force. Study the following examples.
If I were you, I wouldn’t do it. (Here the primary auxiliary were is used to express possibility.)
In spite of this partial overlapping of their functions, the distinction between the primary and modal auxiliaries is a useful one.