]    Do can be used as the main verb of the sentence.

Example:  I do my homework everyday.

 

] When do is used with the 3rd Person Singular subject (he, she and it), it becomes does.

Example:  He does the dishes after dinner. 

] When a sentence needs to change into a question, do can be the auxiliary verb. 

Do comes to help the verb have when the sentence needs to be turned into a question.

Examples:   Do they have a new house? (They have a new house.)
                                     Does she have a brother?  (She has a brother.)

] Short answers to questions need auxiliary do/does.

Do they live in Miami?  Yes, they do.       
Does she work at night?  No, she does not.