Present Simple Tense

To form the Present Simple Tense we use the verb's base form (go, work, speak, study). In 3rd person singular (he, she, it), the base form of the verb takes -s/es. (Auxiliary verbs "be," "do," "have", which can also be used as main verbs, are exceptions.)

Affirmative form

I
you walk
we
they

he/she/it walks /-s/

go - goes /-es/
watch - watches /-es/
play - plays /-s/
study - studies /-es/

! Remember:

to be
I/you/we/they are
he/she/it is

to do
I/you/we/they do
he/she/it does

to have
I/you/we/they have
he/she/it has

Negative form

I
you DO NOT /don't/
we WORK
they

he/she/it DOES NOT
/doesn't / WORK

1. She doesn't often go to the cinema.
2. I don't get up early at the weekend.
3. They don't speak English very well.

Interrogative form

I
DO you WORK?
we
they

DOES he/she/it WORK?

1. Do they speak foreign languages?
2. Do you want a banana?
3. Does your sister play the piano?

Questions and short answers:

Do you like spaghetti?
Yes, I do.
No, I don't.

Does she know Bulgarian?
Yes, she does.
No, she doesn't.
Notes about formation of the 3rd person singular (he, she, it):

If the verb ends in -ss, -sh, -ch, -x or -o, add -es to the base form:
kiss - kisses, finish - finishes, watch - watches, mix - mixes, go - goes

If the verb ends in consonant + y change y to i and add -es:
study - studies, copy - copies, try - tries, carry - carries

We use the Present Simple Tense:
-when we talk about things that happen repeatedly or habitually

With Present Simple Tense we often use time expressions such as always, often, sometimes, usually, seldom, on Saturdays, rarely, never, every day.
-when we talk about permanent or long-lasting situations .
-when we talk about people or things in general
-to indicate general truths, facts and scientific laws
-when we talk about travel plans and timetables (mainly with verbs such as go, leave, arrive, start, come.)
-with state (or stative) verbs such as like, dislike, love, think, seem, look, know, feel, understand, want, need, hate, remember, forget, prefer, believe, mean, taste, hear, see, have (when the meaning is "possess"), own, belong, etc. These verbs are not normally used in the Continuous Tense (but there are exceptions).
The Present Simple Tense is also used in :

sentences after "when", "after", "while", "till" / "until", "before", "as soon as" - When the rain stops, we'll go in the park.
I'll call you back as soon as I arrive home.

zero conditional sentences (when the result of the condition is always true) - If you heat water to 100°C (212°F), it boils.

first conditional sentences (Often called the "real" conditional because it is used for real (or possible) situations. These situations take place if a certain condition is met.) - If you finish your homework I'll bring you to the cinema.
Examples:
John works for 8 hours every day.
Mary lives in London.
Football is a very appreciated sport in Romania.
The sun rises in the east.
Water freezes at 0°C (32°F).
The train leaves in five minutes.
I like ice cream. I don't like milk.
That bicycle belongs to my brother.
Walk down the street to the corner and then turn left.