Present simple auxiliary verbs do

AUXILIARY VERBS DO, DOES, DON'T, DOESN'T IN SIMPLE PRESENT
The auxiliary do is used mainly to form questions and negative sentences with the present simple tense. It is not normally used inaffirmative sentences.
It is also used in forming tag questions and shortened answers. Study the following examples:
'I enjoy cycling in the countryside, but I don't enjoy cycling in towns.'
'Shedoesn't play the ballgames that girls usually play, but she plays football.'
'You don't really like Helen, do you?' 'Of course I do! / No, it's true. I don't.'
'Do all cats drink milk?' 'Most do, somedon't.'
'Doesn't he ever take a day off work?' 'He did once in 1999, but he hasn't so far this year!'


AFFIRMATIVE SENTENCES IN SIMPLE PRESENT

Sólo hay dos formas básicas para el presentesimple; una termina en
–s y la otra no. La estructura de una frase afirmativa en presente simple sería la siguiente:

Sujeto + verbo en presente + complementos
-We read a newspaper every day.-He lives in Madrid.
-Birds fly.
Sólo la TERCERA PERSONA DEL SINGULAR
(he, she and it) lleva -S.

Con la mayoría de los verbos, la tercera persona
del singular se forma simplementeañadiendo una -
S. Sin embargo, con algunos verbos, hay que
añadir -ES o cambiar la terminación del verbo.
Estas son las reglas a seguir:

Verbo
terminado en 3ª personaEjemplo
-ss Añadir –es He passes
-x Añadir –es He fixes-ch Añadir –es He teaches
-sh Añadir –es He washes
-oAñadir –es He goes
Consonante
+ y Cambiar la –y He studies
por...