The doing of something: việc + verb

Việc means "work" or "a matter". Before a verb, it turns the whole action into a noun, like English "-ing" in "finding a flat is hard": việc học tiếng Việt (learning Vietnamese), việc tìm nhà (finding a place to live).

In speech, a verb can already be the subject on its own: Học tiếng Việt không khó. Learning Vietnamese isn't hard. Việc in front adds weight, and it's normal in writing, the news and at work: Việc học tiếng Việt cần nhiều thời gian. Learning Vietnamese takes a lot of time.

It's most useful after another verb or a preposition, where the action becomes the object: Công ty đang xem xét việc tăng lương. The company is considering raising salaries. Anh ấy giỏi trong việc bán hàng. He's good at selling.

Việc names a concrete action that someone does. Sự names a quality or a process, as an idea: việc thay đổi giờ làm (changing the working hours), sự thay đổi (change).

On its own, việc is also simply "a job" or "a task": tìm việc (look for work), Em có việc bận. I've got something on.