Turning a verb or adjective into a noun: sự + word

Sự before a verb or an adjective turns it into a noun for the idea itself, like English "-ment", "-ness" or "-tion": phát triển (develop) becomes sự phát triển (development), giúp đỡ (help) becomes sự giúp đỡ, khác nhau (different) becomes sự khác nhau (the difference).

You need it when that idea is the subject or the object of a sentence, often with của (of) after it: Cảm ơn sự giúp đỡ của anh. Thank you for your help. Sự khác nhau giữa hai từ này là gì? What's the difference between these two words?

It's typical of writing, the news and speeches, and it mostly goes with two-syllable words. In speech, Vietnamese usually keeps the plain verb or adjective: where a report says sự phát triển của thành phố, a friend says Thành phố phát triển nhanh quá. The city's growing so fast. A few sự words are everyday, though, like sự thật (the truth).

Việc before a verb is close, but it names an action someone does: việc học tiếng Việt (learning Vietnamese). Sự names a quality or a process. Cuộc names an event: cuộc họp (a meeting).