|do not have the ending -s with he, she, it
My daughter is four years old and she can read and write.
Mr Johnson can come tomorrow.
|do not require an auxiliary verb (be, do, have)
Can you close the window, please?
BUT: Does he always close this window?
|do not have forms in the past (except "could") and in the future
I must go to the dentist.
BUT: I needed to go to the dentist yesterday.
|do not require "to" before the second (main) verb
We may sit here.
BUT: I want to sit here.