world一字使唔使用大楷?

If the word is a proper noun, then you need to make it capitalized. For example, United Nations / Nokia cellphone / Marks & Spences / George Bush.

Proper nouns = name of specific prsons, places, and things (always capitalized).

So, you don't need to use capital for the word "world" in a sentence.

It is not really. World is not a name. It's only a noun.