In English the shortest possible sentence is a subject and a verb:
I (subject - first person singular nominative personal pronoun, remember?) exist (verb).
Dogs (subject) bark (verb).
They are not fantastically informative sentences, but they are sentences and they are very short!
You have the subject which is what the sentence is about and the verb which tells you what the subject does.
Verbs come in different tenses (a kind of 'time form') so that they can tell you what the subject does in the present or in the future or the past.
The verbs in our mini sentences above are about the present, so they are in the present simple tense. The present simple tells us about things which kind of always happen or never happen...etc.
People always exist, dogs always bark (especially the neighbour's dog).
So for things which happen always, usually, often, sometimes, rarely or never we can use this tense.
In some languages there is a different form of the present for each person too. Greek for example has a different form of the verb for I, you, we, they, he/she/it (first, second and third person singular and plural):
iparcho - I exist
iparcheis - you exist
iparchei - he/she/it exists...
In English the verb only changes for the third person singular, he/she/it. We put an "s" on the verb:
I exist - he exists
Dogs bark - A dog barks
In other words it works like this:
I, you, we, they
he, she, it
VERB + s
Easy enough? Well, there is a catch.
Those sentences up there are all positive. If you want to say what is not always true then you have to use a negative form...and if you want to ask a question about what always happens, you need a question form.
Now, don't panic! Remember auxiliary verbs? Well we use 'do' to make the present simple negative and question forms.
Negative: Cats do (auxiliary verb) not bark (main verb)
Question: Do (auxiliary verb) you exist (main verb)?
In those sentences the 'do' has no meaning, only a function.
[Tip!! The auxiliary verb tells you the tense, the main verb tells you the sense]
So, it looks like this:
...and what about that 's' on the third person form? Well, you can see it goes on the auxiliary verb 'does' and NOT the main verb!
Now if that does not make you agree that English is Easy, then I give up!