subject verb agreementis is correspondence between subject and verb. when the subject  is plural the verb is usually added wit s when the subject is singular the verb is not added with s.
 singular subject : is, he, she, it
plural subject : I and you
auxiliary verb 
do/ does  (in the present simple)
am/is/are/was/were (continouse tenses)
have/has (perfect tenses)
example : the dogs don't want it 
                 the dog does'nt want it
modal verb 
may 
could 
will
must
should
the verb following a modal verb is never in the s form it's always in the infinitive form 
example : my friends might come
                my friend might come