The present tense of a verb designates an action or state of being going on or existing at the time of speaking or writing, as in John walks across the street or She is excited about her new job. Some novelists insist on using the present tense, as if the action were occurring at the moment the reader reads the book. Most irritating, in my opinion.
We can also use the present tense to denote futurity, as in The game is on TV tonight.
We can also use what is called the historical present tense to depict what authors do in their books, as in Fitzgerald paints a somber mood on East Egg.
The third-person singular of a verb requires a special form to create the present tense. Typically, the third-person singular ends in ‑s, ‑es, or ‑ies as in He writes the book, He watches the movie, She verifies the numbers. For all other persons (including third-person plural), the present tense is the same as the base infinitive, as in I write, You write, We write, They write, and so on.
The verb to be forms its present tense using am (first person), are (second person), and is (third person) in the singular and are (all persons) in the plural.