The two types of auxiliary verb are primary verbs and modal verbs. The primary verbs are behave, and do which come in three forms: ending in –s formpast form, and the past participle form as shown in the following table.



Primary verbs be have do
-s form is has does
Past form was/were had did
Past participle been had done




Modal verbs      
will can may shall
would could might should
must ought to used to