The small letter that changes the verb when someone else acts
Why the verb gains one small letter when the subject is a single other person or thing - and nowhere else.
English, never translated
She works in a hospital.
Explanation
In the present simple, the verb changes for only one group of subjects: a single person or thing other than you and me. For those subjects the verb takes the -s ending; with every other subject it keeps its base form. It is one small letter, but listeners notice immediately when it is missing.
My brother plays football every weekend.
The bus leaves at eight o'clock.
I play tennis, but my sister plays better.
It rains a lot here in June.
Negatives and questions already carry the ending inside the helper verb, so the main verb goes back to its base form there. Learners often mark it twice - once on the helper and again on the main verb. Put it on the helper only.