The auxiliary verb DO helps us to make the negative and interrogative form of normal verbs (a few special verbs, like TO BE, CAN, etc) never use DO. Check the videos below to see the other forms.

BUILDING THE ENGLISH SENTENCE

S= subject. V= verb. C= the rest (Complement, Object, Adverbials, etc.)

SPECIAL VERBS (to be, have got, can, will, do, etc.)

+  I am European / I can speak English S + V + C
-   I am not European / I can not* speak English S + V+not + C   
?  Are you European? / Can you speak English? V + S + C (inversion)

* In fact, the negative form of the ver CAN is irregular, we spell it "cannot" or "can't"

NORMAL VERBS (the rest)

+  I live in Spain S + V + C
-   I don't live in Spain S + don't+V + C     (negative with DO)
?  Do you live in Spain? Do + S + V + C   (inversion with DO)