Normally in the present tense we add S to the end of the verb in the 3rd person (He, She, It).

Verb	3rd Person
Speak	Speaks
Play	Plays
Give	Gives
Make	Makes

• He speaks three languages.
• She drinks coffee every morning.
• My dog hates my cat.

Irregular Verbs

Irregular verbs in English in the present tense follow very simple rules. The only change that is made to these verbs is in the third person – for He, She or It.
1. If the verb ends in SS, X, CH, SH or the letter O, we add + ES in the third person.

Verb	3rd Person
Kiss	Kisses
Fix	Fixes
Watch	Watches
Crash	Crashes
Go	Goes

• A mechanic fixes cars.
• She watches soap operas every afternoon.
• He kisses his wife before he goes to work.

2. If the verb ends in a Consonant + Y, we remove the Y and + IES in the third person.

Verb	3rd Person
Carry	Carries
Hurry	Hurries
Study	Studies
Deny	Denies

• Isabel studies every night.
• The baby cries all the time.
• He denies all responsibility.

Negative Sentence

To form the negative we use the auxiliary do not. Again, the only variation occurs in the 3rd person where we use does not.

Positive	Negative
I talk	I do not talk
She talks	She does not talk
You sleep	You do not sleep
He sleeps	He does not sleep
Carol studies	Carol does not study
We study	We do not study

In the negative, the main verb is always in the bare infinitive (without TO). It doesn't change for the third person. We don't put an S on the end of the verb in the negative form. In the examples above - talk, sleep and study do not change in the 3rd person.

• He speaks Italian
  He doesn't speak Italian.

Remember: Do not can be abbreviated to Don't and Does not can be abbreviated to Doesn't.


Source: http://www.grammar.cl/Present/Verbs_Third_Person.htm