“Weakness of attitude becomes weakness of character.” - Albert Einstein

Modal auxiliaries - Verbs that express the degree of certainty of the action in the sentence, or the attitude or opinion of the writer concerning the action.

Modal - Noting or pertaining to mood.

Auxiliary - Additional; supplementary; reserve.

Modal auxiliaries express ideas and feelings. They do not change form for different subjects. For example:

You can read the book. You may read the book. “Can” or “may” denotes permission.

I can read the book. “Can” denotes ability.

You must read the book. “Must” denotes necessity.

Can, could, may, might, must, ought to, shall, should, will, and would are the most common modal auxiliaries.