Pense na sua rotina ex: I wake up, I brush my teeth, I eat, I have a cup of milk, I go to school, I have lunch.
You have lunch, we eat, thay have a piece of cake,
Quando falamos da terceira pessoa no singular acrescentamos o S:
He wakes up, he brushes his hair, he brushes his teeth, he has a cup of milk, he goes to school, he has lunch.
I, you, we, they HAVE lunch, quando usamos He, She, it HAS lunch.