This is the 11881st most frequent Russian word.
заполнять
"To fill in" or "to fill out."
In this sentence, 'заполняю' means 'to fill out,' emphasizing the action of writing or completing information into a form or document.
Я заполняю анкету для участия в проекте.
I am filling out a form to participate in the project.
Here, 'заполняет' means 'fills,' describing the physical act of occupying space, in this case with water.
Река заполняет долину после сильного дождя.
The river fills the valley after a heavy rain.
In this usage, 'заполняет' conveys the action of occupying or bestowing content, specifically text, onto pages.